Y Daniel Liang Introduction to Java Programming Comprehensive Version

INTRODUCTION TO
JAVA
®
PROGRAMMING
COMPREHENSIVE VERSION
Tenth Edition
Y. Daniel Liang
Armstrong Atlantic State University
Boston Columbus Indianapolis New York San Francisco Upper Saddle River
Amsterdam Cape Town Dubai London Madrid Milan Munich Paris Montreal Toronto
Delhi Mexico City Sao Paulo Sydney Hong Kong Seoul Singapore Taipei Tokyo
To Samantha, Michael, and Michelle
Editorial Director, ECS: Marcia Horton
Executive Editor: Tracy Johnson (Dunkelberger)
Editorial Assistant: Jenah Blitz-Stoehr
Director of Marketing: Christy Lesko
Marketing Manager: Yez Alayan
Marketing Assistant: Jon Bryant
Director of Program Management: Erin Gregg
Program Management-Team Lead: Scott Disanno
Program Manager: Carole Snyder
Project Management-Team Lead: Laura Burgess
Project Manager: Robert Engelhardt
Procurement Specialist: Linda Sager
Cover Designer: Marta Samsel
Permissions Supervisor: Michael Joyce
Permissions Administrator: Jenell Forschler
Director, Image Asset Services: Annie Atherton
Manager, Visual Research: Karen Sanatar
Image Permission Coordinator:
Cover Art: © Blend Images—PBNJ Productions/Getty
Images
Media Project Manager: Renata Butera
Full-Service Project Management: Haseen Khan,
Laserwords Pvt Ltd
Credits and acknowledgments borrowed from other sources and reproduced, with permission, in this textbook
appear on the appropriate page within text.
Microsoft® and Windows® are registered trademarks of the Microsoft Corporation in the U.S.A. and other
countries. Screen shots and icons reprinted with permission from the Microsoft Corporation. This book is not
sponsored or endorsed by or affiliated with the Microsoft Corporation.
Copyright © 2015, 2013, 2011 Pearson Education, Inc., publishing as Prentice Hall, 1 Lake Street, Upper Saddle
River, New Jersey, 07458. All rights reserved. Printed in the United States of America. This publication is protected
by Copyright, and permission should be obtained from the publisher prior to any prohibited reproduction, storage in
a retrieval system, or transmission in any form or by any means, electronic, mechanical, photocopying, recording,
or likewise. To obtain permission(s) to use material from this work, please submit a written request to Pearson
Education, Inc., Permissions Department, One Lake Street, Upper Saddle River, New Jersey 07458, or you may fax
your request to 201-236-3290.
Many of the designations by manufacturers and sellers to distinguish their products are claimed as trademarks.
Where those designations appear in this book, and the publisher was aware of a trademark claim, the designations
have been printed in initial caps or all caps.
Library of Congress Cataloging-in-Publication Data available upon request.
10 9 8 7 6 5 4 3 2 1
ISBN 10: 0-13-376131-2
ISBN 13: 978-0-13-376131-3
PREFACE
Dear Reader,
Many of you have provided feedback on earlier editions of this book, and your comments and
suggestions have greatly improved the book. This edition has been substantially enhanced in
presentation, organization, examples, exercises, and supplements. The new edition:
■
Replaces Swing with JavaFX. JavaFX is a new framework for developing Java GUI programs. JavaFX greatly simplifies GUI programming and is easier to learn than Swing.
■
Introduces exception handling, abstract classes, and interfaces before GUI programming to
enable the GUI chapters to be skipped completely if the instructor chooses not to cover GUI.
■
Covers introductions to objects and strings earlier in Chapter 4 to enable students to use
objects and strings to develop interesting programs early.
■
Includes many new interesting examples and exercises to stimulate student interests. More
than 100 additional programming exercises are provided to instructors only on the Companion Website.
Please visit www.pearsonhighered.com/liang for a complete list of new features as well as
correlations to the previous edition.
The book is fundamentals first by introducing basic programming concepts and techniques
before designing custom classes. The fundamental concepts and techniques of selection
statements, loops, methods, and arrays are the foundation for programming. Building this
strong foundation prepares students to learn object-oriented programming and advanced Java
programming.
This book teaches programming in a problem-driven way that focuses on problem solving rather than syntax. We make introductory programming interesting by using thoughtprovoking problems in a broad context. The central thread of early chapters is on problem
solving. Appropriate syntax and library are introduced to enable readers to write programs for
solving the problems. To support the teaching of programming in a problem-driven way, the
book provides a wide variety of problems at various levels of difficulty to motivate students.
To appeal to students in all majors, the problems cover many application areas, including
math, science, business, financial, gaming, animation, and multimedia.
The book seamlessly integrates programming, data structures, and algorithms into one text.
It employs a practical approach to teach data structures. We first introduce how to use various
data structures to develop efficient algorithms, and then show how to implement these data
structures. Through implementation, students gain a deep understanding on the efficiency of
data structures and on how and when to use certain data structures. Finally we design and
implement custom data structures for trees and graphs.
The book is widely used in the introductory programming, data structures, and algorithms
courses in the universities around the world. This comprehensive version covers fundamentals
of programming, object-oriented programming, GUI programming, data structures, algorithms,
concurrency, networking, database, and Web programming. It is designed to prepare students
to become proficient Java programmers. A brief version (Introduction to Java Programming,
Brief Version, Tenth Edition) is available for a first course on programming, commonly known
as CS1. The brief version contains the first 18 chapters of the comprehensive version. The first
13 chapters are appropriate for preparing the AP Computer Science exam.
The best way to teach programming is by example, and the only way to learn programming is by doing. Basic concepts are explained by example and a large number of exercises
what is new?
fundamentals-first
problem-driven
data structures
comprehensive version
brief version
AP Computer Science
examples and exercises
iii
iv Preface
with various levels of difficulty are provided for students to practice. For our programming
courses, we assign programming exercises after each lecture.
Our goal is to produce a text that teaches problem solving and programming in a broad
context using a wide variety of interesting examples. If you have any comments on and suggestions for improving the book, please email me.
Sincerely,
Y. Daniel Liang
y.daniel.liang@gmail.com
www.cs.armstrong.edu/liang
www.pearsonhighered.com/liang
ACM/IEEE Curricular 2013 and ABET
Course Assessment
The new ACM/IEEE Computer Science Curricular 2013 defines the Body of Knowledge
organized into 18 Knowledge Areas. To help instructors design the courses based on this book,
we provide sample syllabi to identify the Knowledge Areas and Knowledge Units. The sample
syllabi are for a three semester course sequence and serve as an example for institutional customization. The sample syllabi are available to instructors at www.pearsonhighered.com/liang.
Many of our users are from the ABET-accredited programs. A key component of the ABET
accreditation is to identify the weakness through continuous course assessment against the course
outcomes. We provide sample course outcomes for the courses and sample exams for measuring
course outcomes on the instructor Website accessible from www.pearsonhighered.com/liang.
What’s New in This Edition?
This edition is completely revised in every detail to enhance clarity, presentation, content,
examples, and exercises. The major improvements are as follows:
■
Updated to Java 8.
■
Since Swing is replaced by JavaFX, all GUI examples and exercises are revised using
JavaFX.
■
Lambda expressions are used to simplify coding in JavaFX and threads.
■
More than 100 additional programming exercises with solutions are provided to the
instructor on the Companion Website. These exercises are not printed in the text.
■
Math methods are introduced earlier in Chapter 4 to enable students to write code using
math functions.
■
Strings are introduced earlier in Chapter 4 to enable students to use objects and strings to
develop interesting programs early.
■
The GUI chapters are moved to after abstract classes and interfaces so that these chapters
can be easily skipped if the instructor chooses not to cover GUI.
■
Chapters 4, 14, 15, and 16 are brand new chapters.
■
Chapters 28 and 29 have been substantially revised with simpler implementations for minimum spanning trees and shortest paths.
Preface v
Pedagogical Features
The book uses the following elements to help students get the most from the material:
■
The Objectives at the beginning of each chapter list what students should learn from the
chapter. This will help them determine whether they have met the objectives after completing
the chapter.
■
The Introduction opens the discussion with representative problems to give the reader an
overview of what to expect from the chapter.
■
Key Points highlight the important concepts covered in each section.
■
Check Points provide review questions to help students track their progress as they read
through the chapter and evaluate their learning.
■
Problems and Case Studies, carefully chosen and presented in an easy-to-follow style,
teach problem solving and programming concepts. The book uses many small, simple, and
stimulating examples to demonstrate important ideas.
■
The Chapter Summary reviews the important subjects that students should understand and remember. It helps them reinforce the key concepts they have learned in the
chapter.
■
Quizzes are accessible online, grouped by sections, for students to do self-test on programming concepts and techniques.
■
Programming Exercises are grouped by sections to provide students with opportunities
to apply the new skills they have learned on their own. The level of difficulty is rated as
easy (no asterisk), moderate (*), hard (**), or challenging (***). The trick of learning
programming is practice, practice, and practice. To that end, the book provides a great
many exercises. Additionally, more than 100 programming exercises with solutions are
provided to the instructors on the Companion Website. These exercises are not printed in
the text.
■
Notes, Tips, Cautions, and Design Guides are inserted throughout the text to offer valuable advice and insight on important aspects of program development.
Note
Provides additional information on the subject and reinforces important concepts.
Tip
Teaches good programming style and practice.
Caution
Helps students steer away from the pitfalls of programming errors.
Design Guide
Provides guidelines for designing programs.
Flexible Chapter Orderings
The book is designed to provide flexible chapter orderings to enable GUI, exception handling,
recursion, generics, and the Java Collections Framework to be covered earlier or later. The
diagram on the next page shows the chapter dependencies.
vi Preface
Part I: Fundamentals of
Programming
Chapter 1 Introduction to
Computers, Programs, and
Java
Chapter 2 Elementary
Programming
Chapter 3 Selections
Part II: Object-Oriented
Programming
Part IV: Data Structures and
Algorithms
Part III: GUI Programming
Chapter 9 Objects and Classes
Chapter 14 JavaFX Basics
Ch 7
Chapter 18 Recursion
Chapter 10 Thinking in Objects
Chapter 15 Event-Driven
Programming and
Animations
Ch 13
Chapter 19 Generics
Chapter 11 Inheritance and
Polymorphism
Chapter 12 Exception
Handling and Text I/O
Chapter 4 Mathematical
Functions, Characters,
and Strings
Chapter 13 Abstract Classes
and Interfaces
Chapter 5 Loops
Chapter 17 Binary I/O
Chapter 16 JavaFX Controls
and Multimedia
Ch 16
Chapter 30 Multithreading and
Parallel Programming
Chapter 31 Networking
Chapter 20 Lists, Stacks, Queues,
and Priority Queues
Chapter 32 Java Database
Programming
Chapter 21 Sets and Maps
Chapter 33 JavaServer Faces
Chapter 34 Advanced GUI
Programming
Chapter 22 Developping
Efficient Algorithms
Chapter 35 Advanced Database
Programming
Chapter 23 Sorting
Chapter 6 Methods
Chapter 24 Implementing Lists,
Stacks, Queues, and Priority
Queues
Chapter 7 Single-Dimensional
Arrays
Chapter 25 Binary Search Trees
Chapter 8 Multidimensional
Arrays
Part V: Advanced Java
Programming
Chapter 36 Internationalization
Chapter 37 Servlets
Note: Chapters 1–18 are in the
brief version of this book.
Chapter 26 AVL Trees
Note: Chapters 1–33 are in the
comprehensive version.
Chapter 27 Hashing
Note: Chapters 34–42 are bonus
chapters available from the
Companion Website.
Chapter 28 Graphs and
Applications
Chapter 38 JavaServer Pages
Chapter 39 Web Services
Chapter 29 Weighted Graphs
and Applications
Chapter 40 2-4 Trees and BTrees
Chapter 41 Red-Black Trees
Ch 9
Chapter 42 Testing Using JUnit
Preface vii
Organization of the Book
The chapters can be grouped into five parts that, taken together, form a comprehensive introduction to Java programming, data structures and algorithms, and database and Web programming.
Because knowledge is cumulative, the early chapters provide the conceptual basis for understanding programming and guide students through simple examples and exercises; subsequent
chapters progressively present Java programming in detail, culminating with the development
of comprehensive Java applications. The appendixes contain a mixed bag of topics, including an
introduction to number systems, bitwise operations, regular expressions, and enumerated types.
Part I: Fundamentals of Programming (Chapters 1–8)
The first part of the book is a stepping stone, preparing you to embark on the journey of learning
Java. You will begin to learn about Java (Chapter 1) and fundamental programming techniques
with primitive data types, variables, constants, assignments, expressions, and operators (Chapter 2),
selection statements (Chapter 3), mathematical functions, characters, and strings (Chapter 4), loops
(Chapter 5), methods (Chapter 6), and arrays (Chapters 7–8). After Chapter 7, you can jump to
Chapter 18 to learn how to write recursive methods for solving inherently recursive problems.
Part II: Object-Oriented Programming (Chapters 9–13, and 17)
This part introduces object-oriented programming. Java is an object-oriented programming
language that uses abstraction, encapsulation, inheritance, and polymorphism to provide
great flexibility, modularity, and reusability in developing software. You will learn programming with objects and classes (Chapters 9–10), class inheritance (Chapter 11), polymorphism
(Chapter 11), exception handling (Chapter 12), abstract classes (Chapter 13), and interfaces
(Chapter 13). Text I/O is introduced in Chapter 12 and binary I/O is discussed in Chapter 17.
Part III: GUI Programming (Chapters 14–16 and Bonus Chapter 34)
JavaFX is a new framework for developing Java GUI programs. It is not only useful for
developing GUI programs, but also an excellent pedagogical tool for learning object-oriented
programming. This part introduces Java GUI programming using JavaFX in Chapters 14–16.
Major topics include GUI basics (Chapter 14), container panes (Chapter 14), drawing shapes
(Chapter 14), event-driven programming (Chapter 15), animations (Chapter 15), and GUI
controls (Chapter 16), and playing audio and video (Chapter 16). You will learn the architecture of JavaFX GUI programming and use the controls, shapes, panes, image, and video to
develop useful applications. Chapter 34 covers advanced features in JavaFX.
Part IV: Data Structures and Algorithms (Chapters 18–29 and Bonus Chapters 40–41)
This part covers the main subjects in a typical data structures and algorithms course. Chapter 18
introduces recursion to write methods for solving inherently recursive problems. Chapter 19
presents how generics can improve software reliability. Chapters 20 and 21 introduce the
Java Collection Framework, which defines a set of useful API for data structures. Chapter 22
discusses measuring algorithm efficiency in order to choose an appropriate algorithm for
applications. Chapter 23 describes classic sorting algorithms. You will learn how to implement
several classic data structures lists, queues, and priority queues in Chapter 24. Chapters 25 and
26 introduce binary search trees and AVL trees. Chapter 27 presents hashing and implementing maps and sets using hashing. Chapters 28 and 29 introduce graph applications. The 2-4
trees, B-trees, and red-black trees are covered in Bonus Chapters 40–41.
Part V: Advanced Java Programming (Chapters 30–33 and Bonus Chapters 35–39, 42)
This part of the book is devoted to advanced Java programming. Chapter 30 treats the use
of multithreading to make programs more responsive and interactive and introduces parallel
programming. Chapter 31 discusses how to write programs that talk with each other from
different hosts over the Internet. Chapter 32 introduces the use of Java to develop database
viii Preface
projects. Chapter 33 introduces modern Web application development using JavaServer Faces.
Chapter 35 delves into advanced Java database programming. Chapter 36 covers the use of
internationalization support to develop projects for international audiences. Chapters 37 and
38 introduce how to use Java servlets and JavaServer Pages to generate dynamic content from
Web servers. Chapter 39 discusses Web services. Chapter 42 introduces testing Java programs
using JUnit.
Appendixes
This part of the book covers a mixed bag of topics. Appendix A lists Java keywords.
Appendix B gives tables of ASCII characters and their associated codes in decimal and in
hex. Appendix C shows the operator precedence. Appendix D summarizes Java modifiers and
their usage. Appendix E discusses special floating-point values. Appendix F introduces number systems and conversions among binary, decimal, and hex numbers. Finally, Appendix G
introduces bitwise operations. Appendix H introduces regular expressions. Appendix I covers
enumerated types.
Java Development Tools
IDE tutorials
You can use a text editor, such as the Windows Notepad or WordPad, to create Java programs
and to compile and run the programs from the command window. You can also use a Java
development tool, such as NetBeans or Eclipse. These tools support an integrated development environment (IDE) for developing Java programs quickly. Editing, compiling, building,
executing, and debugging programs are integrated in one graphical user interface. Using these
tools effectively can greatly increase your programming productivity. NetBeans and Eclipse
are easy to use if you follow the tutorials. Tutorials on NetBeans and Eclipse can be found
under Tutorials on the Student Companion Website at www.pearsonhighered.com/liang.
Student Resource Website
The Student Resource Website www.pearsonhighered.com/liang provides access to some of the
following resources. Other resources are available using the student access code printed on the
inside front cover of this book. (For students with a used copy of this book, you can purchase
access to the premium student resources through www.pearsonhighered.com/liang.)
■
Answers to review questions
■
Solutions to even-numbered programming exercises
■
Source code for the examples in the book
■
Interactive quiz (organized by sections for each chapter)
■
Supplements
■
Debugging tips
■
Algorithm animations
■
Errata
Instructor Resource Website
The Instructor Resource Website, accessible from www.pearsonhighered.com/liang, provides
access to the following resources:
■
Microsoft PowerPoint slides with interactive buttons to view full-color, syntax-highlighted
source code and to run programs without leaving the slides.
■
Solutions to all programming exercises. Students will have access to the solutions of evennumbered programming exercises.
Preface ix
■
More than 100 additional programming exercises organized by chapters. These exercises
are available only to the instructors. Solutions to these exercises are provided.
■
Web-based quiz generator. (Instructors can choose chapters to generate quizzes from a
large database of more than two thousand questions.)
■
Sample exams. Most exams have four parts:
■
Multiple-choice questions or short-answer questions
■
Correct programming errors
■
Trace programs
■
Write programs
■
ACM/IEEE Curricula 2013. The new ACM/IEEE Computer Science Curricula 2013
defines the Body of Knowledge organized into 18 Knowledge Areas. To help instructors
design the courses based on this book, we provide sample syllabi to identify the Knowledge Areas and Knowledge Units. The sample syllabi are for a three semester course
sequence and serve as an example for institutional customization. Instructors can access
the syllabi at www.pearsonhighered.com/liang.
■
Sample exams with ABET course assessment.
■
Projects. In general, each project gives a description and asks students to analyze, design,
and implement the project.
Some readers have requested the materials from the Instructor Resource Website. Please
understand that these are for instructors only. Such requests will not be answered.
Online Practice and Assessment
with MyProgrammingLab
MyProgrammingLab helps students fully grasp the logic, semantics, and syntax of programming. Through practice exercises and immediate, personalized feedback, MyProgrammingLab improves the programming competence of beginning students who often struggle
with the basic concepts and paradigms of popular high-level programming languages.
A self-study and homework tool, a MyProgrammingLab course consists of hundreds of
small practice problems organized around the structure of this textbook. For students, the system automatically detects errors in the logic and syntax of their code submissions and offers
targeted hints that enable students to figure out what went wrong—and why. For instructors,
a comprehensive gradebook tracks correct and incorrect answers and stores the code inputted
by students for review.
MyProgrammingLab is offered to users of this book in partnership with Turing’s Craft, the
makers of the CodeLab interactive programming exercise system. For a full demonstration,
to see feedback from instructors and students, or to get started using MyProgrammingLab in
your course, visit www.myprogramminglab.com.
VideoNotes
We are excited about the new VideoNotes feature that is found in this new edition. These
videos provide additional help by presenting examples of key topics and showing how to
solve problems completely, from design through coding. VideoNotes are available from
www.pearsonhighered.com/liang.
VideoNote
x Preface
Algorithm Animations
Animation
We have provided numerous animations for algorithms. These are valuable pedagogical tools
to demonstrate how algorithms work. Algorithm animations can be accessed from the Companion Website.
Acknowledgments
I would like to thank Armstrong Atlantic State University for enabling me to teach what I
write and for supporting me in writing what I teach. Teaching is the source of inspiration for
continuing to improve the book. I am grateful to the instructors and students who have offered
comments, suggestions, bug reports, and praise.
This book has been greatly enhanced thanks to outstanding reviews for this and previous
editions. The reviewers are: Elizabeth Adams (James Madison University), Syed Ahmed (North
Georgia College and State University), Omar Aldawud (Illinois Institute of Technology), Stefan
Andrei (Lamar University), Yang Ang (University of Wollongong, Australia), Kevin Bierre
(Rochester Institute of Technology), David Champion (DeVry Institute), James Chegwidden
(Tarrant County College), Anup Dargar (University of North Dakota), Charles Dierbach (Towson
University), Frank Ducrest (University of Louisiana at Lafayette), Erica Eddy (University of
Wisconsin at Parkside), Deena Engel (NewYork University), Henry A. Etlinger (Rochester Institute
of Technology), James Ten Eyck (Marist College), Myers Foreman (Lamar University), Olac
Fuentes (University of Texas at El Paso), Edward F. Gehringer (North Carolina State University),
Harold Grossman (Clemson University), Barbara Guillot (Louisiana State University), Stuart
Hansen (University of Wisconsin, Parkside), Dan Harvey (Southern Oregon University), Ron
Hofman (Red River College, Canada), Stephen Hughes (Roanoke College), Vladan Jovanovic
(Georgia Southern University), Edwin Kay (Lehigh University), Larry King (University of
Texas at Dallas), Nana Kofi (Langara College, Canada), George Koutsogiannakis (Illinois
Institute of Technology), Roger Kraft (Purdue University at Calumet), Norman Krumpe (Miami
University), Hong Lin (DeVry Institute), Dan Lipsa (Armstrong Atlantic State University),
James Madison (Rensselaer Polytechnic Institute), Frank Malinowski (Darton College),
Tim Margush (University of Akron), Debbie Masada (Sun Microsystems), Blayne Mayfield
(Oklahoma State University), John McGrath (J.P. McGrath Consulting), Hugh McGuire (Grand
Valley State), Shyamal Mitra (University of Texas at Austin), Michel Mitri (James Madison
University), Kenrick Mock (University of Alaska Anchorage), Frank Murgolo (California
State University, Long Beach), Jun Ni (University of Iowa), Benjamin Nystuen (University of
Colorado at Colorado Springs), Maureen Opkins (CA State University, Long Beach), Gavin
Osborne (University of Saskatchewan), Kevin Parker (Idaho State University), Dale Parson
(Kutztown University), Mark Pendergast (Florida Gulf Coast University), Richard Povinelli
(Marquette University), Roger Priebe (University of Texas at Austin), Mary Ann Pumphrey (De
Anza Junior College), Pat Roth (Southern Polytechnic State University), Amr Sabry (Indiana
University), Ben Setzer (Kennesaw State University), Carolyn Schauble (Colorado State
University), David Scuse (University of Manitoba), Ashraf Shirani (San Jose State University),
Daniel Spiegel (Kutztown University), Joslyn A. Smith (Florida Atlantic University), Lixin
Tao (Pace University), Ronald F. Taylor (Wright State University), Russ Tront (Simon Fraser
University), Deborah Trytten (University of Oklahoma), Michael Verdicchio (Citadel), Kent
Vidrine (George Washington University), and Bahram Zartoshty (California State University
at Northridge).
It is a great pleasure, honor, and privilege to work with Pearson. I would like to thank Tracy
Johnson and her colleagues Marcia Horton, Yez Alayan, Carole Snyder, Scott Disanno, Bob
Engelhardt, Haseen Khan, and their colleagues for organizing, producing, and promoting this
project.
As always, I am indebted to my wife, Samantha, for her love, support, and encouragement.
BRIEF CONTENTS
1 Introduction to Computers, Programs,
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
and Java
Elementary Programming
Selections
Mathematical Functions, Characters,
and Strings
Loops
Methods
Single-Dimensional Arrays
Multidimensional Arrays
Objects and Classes
Object-Oriented Thinking
Inheritance and Polymorphism
Exception Handling and Text I/O
Abstract Classes and Interfaces
JavaFX Basics
Event-Driven Programming
and Animations
JavaFX UI Controls and Multimedia
Binary I/O
Recursion
Generics
Lists, Stacks, Queues,
and Priority Queues
Sets and Maps
Developing Efficient Algorithms
Sorting
Implementing Lists, Stacks, Queues,
and Priority Queues
Binary Search Trees
AVL Trees
Hashing
Graphs and Applications
1
33
75
119
157
203
245
287
321
365
409
449
495
535
585
629
677
705
737
761
797
821
861
895
929
965
985
1015
29
30
31
32
33
Weighted Graphs and Applications
1061
Multithreading and Parallel Programming 1097
Networking
1139
Java Database Programming
1173
JavaServer Faces
1213
Chapters 34–42 are bonus Web chapters
34-1
34 Advanced JavaFX
35 Advanced Database Programming
35-1
36 Internationalization
36-1
37 Servlets
37-1
38 JavaServer Pages
38-1
39 Web Services
39-1
40 2-4 Trees and B-Trees
40-1
41 Red-Black Trees
41-1
42 Testing Using JUnit
42-1
Appendixes
A
B
C
D
E
F
G
H
I
Java Keywords
The ASCII Character Set
Operator Precedence Chart
Java Modifiers
Special Floating-Point Values
Number Systems
Bitwise Operatoirns
Regular Expressions
Enumerated Types
Index
1263
1266
1268
1270
1272
1273
1277
1278
1283
1289
xi
CONTENTS
Chapter 1 Introduction to Computers, Programs,
and Java
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
1.9
1.10
1.11
1.12
Introduction
What Is a Computer?
Programming Languages
Operating Systems
Java, the World Wide Web, and Beyond
The Java Language Specification, API, JDK, and IDE
A Simple Java Program
Creating, Compiling, and Executing a Java Program
Programming Style and Documentation
Programming Errors
Developing Java Programs Using NetBeans
Developing Java Programs Using Eclipse
Chapter 2 Elementary Programming
2.1
2.2
2.3
2.4
2.5
2.6
2.7
2.8
2.9
2.10
2.11
2.12
2.13
2.14
2.15
2.16
2.17
2.18
Introduction
Writing a Simple Program
Reading Input from the Console
Identifiers
Variables
Assignment Statements and Assignment Expressions
Named Constants
Naming Conventions
Numeric Data Types and Operations
Numeric Literals
Evaluating Expressions and Operator Precedence
Case Study: Displaying the Current Time
Augmented Assignment Operators
Increment and Decrement Operators
Numeric Type Conversions
Software Development Process
Case Study: Counting Monetary Units
Common Errors and Pitfalls
Chapter 3 Selections
3.1
3.2
3.3
3.4
3.5
3.6
3.7
3.8
3.9
3.10
3.11
3.12
3.13
3.14
xii
Introduction
boolean Data Type
if Statements
Two-Way if-else Statements
Nested if and Multi-Way if-else Statements
Common Errors and Pitfalls
Generating Random Numbers
Case Study: Computing Body Mass Index
Case Study: Computing Taxes
Logical Operators
Case Study: Determining Leap Year
Case Study: Lottery
switch Statements
Conditional Expressions
1
2
2
7
9
10
11
12
15
18
20
23
25
33
34
34
37
39
40
41
43
44
44
48
50
52
54
55
56
59
63
65
75
76
76
78
80
81
83
87
89
90
93
97
98
100
103
xiii
Operator Precedence and Associativity
Debugging
104
106
Chapter 4 Mathematical Functions, Characters,
and Strings
119
3.15
3.16
4.1
4.2
4.3
4.4
4.5
4.6
Introduction
Common Mathematical Functions
Character Data Type and Operations
The String Type
Case Studies
Formatting Console Output
Chapter 5 Loops
5.1
5.2
5.3
5.4
5.5
5.6
5.7
5.8
5.9
5.10
5.11
Introduction
The while Loop
The do-while Loop
The for Loop
Which Loop to Use?
Nested Loops
Minimizing Numeric Errors
Case Studies
Keywords break and continue
Case Study: Checking Palindromes
Case Study: Displaying Prime Numbers
Chapter 6 Methods
6.1
6.2
6.3
6.4
6.5
6.6
6.7
6.8
6.9
6.10
6.11
Introduction
Defining a Method
Calling a Method
void Method Example
Passing Arguments by Values
Modularizing Code
Case Study: Converting Hexadecimals to Decimals
Overloading Methods
The Scope of Variables
Case Study: Generating Random Characters
Method Abstraction and Stepwise Refinement
Chapter 7 Single-Dimensional Arrays
7.1
7.2
7.3
7.4
7.5
7.6
7.7
7.8
7.9
7.10
7.11
7.12
7.13
Introduction
Array Basics
Case Study: Analyzing Numbers
Case Study: Deck of Cards
Copying Arrays
Passing Arrays to Methods
Returning an Array from a Method
Case Study: Counting the Occurrences of Each Letter
Variable-Length Argument Lists
Searching Arrays
Sorting Arrays
The Arrays Class
Command-Line Arguments
Chapter 8 Multidimensional Arrays
8.1
8.2
Introduction
Two-Dimensional Array Basics
120
120
125
130
139
145
157
158
158
168
170
174
176
178
179
184
187
188
203
204
204
206
209
212
215
217
219
222
223
225
245
246
246
253
254
256
257
260
261
264
265
269
270
272
287
288
288
xiv Contents
8.3
8.4
8.5
8.6
8.7
8.8
Processing Two-Dimensional Arrays
Passing Two-Dimensional Arrays to Methods
Case Study: Grading a Multiple-Choice Test
Case Study: Finding the Closest Pair
Case Study: Sudoku
Multidimensional Arrays
Chapter 9 Objects and Classes
9.1
9.2
9.3
9.4
9.5
9.6
9.7
9.8
9.9
9.10
9.11
9.12
9.13
9.14
Introduction
Defining Classes for Objects
Example: Defining Classes and Creating Objects
Constructing Objects Using Constructors
Accessing Objects via Reference Variables
Using Classes from the Java Library
Static Variables, Constants, and Methods
Visibility Modifiers
Data Field Encapsulation
Passing Objects to Methods
Array of Objects
Immutable Objects and Classes
The Scope of Variables
The this Reference
Chapter 10 Object-Oriented Thinking
10.1
10.2
10.3
10.4
10.5
10.6
10.7
10.8
10.9
10.10
10.11
Introduction
Class Abstraction and Encapsulation
Thinking in Objects
Class Relationships
Case Study: Designing the Course Class
Case Study: Designing a Class for Stacks
Processing Primitive Data Type Values as Objects
Automatic Conversion between Primitive Types
and Wrapper Class Types
The BigInteger and BigDecimal Classes
The String Class
The StringBuilder and StringBuffer Classes
Chapter 11 Inheritance and Polymorphism
11.1
11.2
11.3
11.4
11.5
11.6
11.7
11.8
11.9
11.10
11.11
11.12
11.13
11.14
11.15
Introduction
Superclasses and Subclasses
Using the super Keyword
Overriding Methods
Overriding vs. Overloading
The Object Class and Its toString() Method
Polymorphism
Dynamic Binding
Casting Objects and the instanceof Operator
The Object’s equals Method
The ArrayList Class
Useful Methods for Lists
Case Study: A Custom Stack Class
The protected Data and Methods
Preventing Extending and Overriding
Chapter 12 Exception Handling and Text I/O
12.1
12.2
Introduction
Exception-Handling Overview
291
293
294
296
298
301
321
322
322
324
329
330
334
337
342
344
347
351
353
355
356
365
366
366
370
373
376
378
380
383
384
386
392
409
410
410
416
419
420
422
423
424
427
431
432
438
439
440
442
449
450
450
Contents xv
12.3
12.4
12.5
12.6
12.7
12.8
12.9
12.10
12.11
12.12
12.13
Exception Types
More on Exception Handling
The finally Clause
When to Use Exceptions
Rethrowing Exceptions
Chained Exceptions
Defining Custom Exception Classes
The File Class
File Input and Output
Reading Data from the Web
Case Study: Web Crawler
Chapter 13 Abstract Classes and Interfaces
13.1
13.2
13.3
13.4
13.5
13.6
13.7
13.8
13.9
13.10
Introduction
Abstract Classes
Case Study: the Abstract Number Class
Case Study: Calendar and GregorianCalendar
Interfaces
The Comparable Interface
The Cloneable Interface
Interfaces vs. Abstract Classes
Case Study: The Rational Class
Class Design Guidelines
Chapter 14 JavaFX Basics
14.1
14.2
14.3
14.4
14.5
14.6
14.7
14.8
14.9
14.10
14.11
14.12
Introduction
JavaFX vs Swing and AWT
The Basic Structure of a JavaFX Program
Panes, UI Controls, and Shapes
Property Binding
Common Properties and Methods for Nodes
The Color Class
The Font Class
The Image and ImageView Classes
Layout Panes
Shapes
Case Study: The ClockPane Class
Chapter 15 Event-Driven Programming
and Animations
15.1
15.2
15.3
15.4
15.5
15.6
15.7
15.8
15.9
15.10
15.11
15.12
Introduction
Events and Event Sources
Registering Handlers and Handling Events
Inner Classes
Anonymous Inner Class Handlers
Simplifying Event Handling Using Lambda Expressions
Case Study: Loan Calculator
Mouse Events
Key Events
Listeners for Observable Objects
Animation
Case Study: Bouncing Ball
Chapter 16 JavaFX UI Controls and Multimedia
16.1
16.2
Introduction
Labeled and Label
455
458
466
467
468
469
470
473
476
482
484
495
496
496
501
503
506
509
513
517
520
525
535
536
536
536
539
542
545
546
547
549
552
560
572
585
586
588
589
593
594
597
600
602
603
606
608
616
629
630
630
xvi Contents
16.3
16.4
16.5
16.6
16.7
16.8
16.9
16.10
16.11
16.12
16.13
16.14
Button
CheckBox
RadioButton
TextField
TextArea
ComboBox
ListView
ScrollBar
Slider
Case Study: Developing a Tic-Tac-Toe Game
Video and Audio
Case Study: National Flags and Anthems
Chapter 17 Binary I/O
17.1
17.2
17.3
17.4
17.5
17.6
17.7
Introduction
How Is Text I/O Handled in Java?
Text I/O vs. Binary I/O
Binary I/O Classes
Case Study: Copying Files
Object I/O
Random-Access Files
Chapter 18 Recursion
18.1
18.2
18.3
18.4
18.5
18.6
18.7
18.8
18.9
18.10
Introduction
Case Study: Computing Factorials
Case Study: Computing Fibonacci Numbers
Problem Solving Using Recursion
Recursive Helper Methods
Case Study: Finding the Directory Size
Case Study: Tower of Hanoi
Case Study: Fractals
Recursion vs. Iteration
Tail Recursion
Chapter 19 Generics
19.1
19.2
19.3
19.4
19.5
19.6
19.7
19.8
19.9
Introduction
Motivations and Benefits
Defining Generic Classes and Interfaces
Generic Methods
Case Study: Sorting an Array of Objects
Raw Types and Backward Compatibility
Wildcard Generic Types
Erasure and Restrictions on Generics
Case Study: Generic Matrix Class
Chapter 20 Lists, Stacks, Queues,
and Priority Queues
20.1
20.2
20.3
20.4
20.5
20.6
20.7
20.8
Introduction
Collections
Iterators
Lists
The Comparator Interface
Static Methods for Lists and Collections
Case Study: Bouncing Balls
Vector and Stack Classes
632
634
637
639
641
644
647
651
654
657
662
665
677
678
678
679
680
691
692
697
705
706
706
709
712
714
717
719
722
726
727
737
738
738
740
742
744
746
747
750
752
761
762
762
766
767
772
773
777
781
Contents xvii
20.9
20.10
Queues and Priority Queues
Case Study: Evaluating Expressions
Chapter 21 Sets and Maps
783
786
797
Introduction
Sets
Comparing the Performance of Sets and Lists
Case Study: Counting Keywords
Maps
Case Study: Occurrences of Words
Singleton and Unmodifiable Collections and Maps
798
798
806
809
810
815
816
Chapter 22 Developing Efficient Algorithms
821
21.1
21.2
21.3
21.4
21.5
21.6
21.7
22.1
22.2
22.3
22.4
22.5
22.6
22.7
22.8
22.9
22.10
Introduction
Measuring Algorithm Efficiency Using Big O Notation
Examples: Determining Big O
Analyzing Algorithm Time Complexity
Finding Fibonacci Numbers Using Dynamic Programming
Finding Greatest Common Divisors Using Euclid’s Algorithm
Efficient Algorithms for Finding Prime Numbers
Finding the Closest Pair of Points Using Divide-and-Conquer
Solving the Eight Queens Problem Using Backtracking
Computational Geometry: Finding a Convex Hull
Chapter 23 Sorting
23.1
23.2
23.3
23.4
23.5
23.6
23.7
23.8
Introduction
Insertion Sort
Bubble Sort
Merge Sort
Quick Sort
Heap Sort
Bucket Sort and Radix Sort
External Sort
Chapter 24 Implementing Lists, Stacks, Queues,
and Priority Queues
24.1
24.2
24.3
24.4
24.5
24.6
Introduction
Common Features for Lists
Array Lists
Linked Lists
Stacks and Queues
Priority Queues
Chapter 25 Binary Search Trees
25.1
25.2
25.3
25.4
25.5
25.6
Introduction
Binary Search Trees
Deleting Elements from a BST
Tree Visualization and MVC
Iterators
Case Study: Data Compression
Chapter 26 AVL Trees
26.1
26.2
26.3
Introduction
Rebalancing Trees
Designing Classes for AVL Trees
822
822
824
828
831
833
837
843
846
849
861
862
862
864
867
870
874
881
883
895
896
896
900
906
920
924
929
930
930
943
949
952
954
965
966
966
969
xviii Contents
26.4
26.5
26.6
26.7
26.8
26.9
Overriding the insert Method
Implementing Rotations
Implementing the delete Method
The AVLTree Class
Testing the AVLTree Class
AVL Tree Time Complexity Analysis
Chapter 27 Hashing
27.1
27.2
27.3
27.4
27.5
27.6
27.7
27.8
Introduction
What Is Hashing?
Hash Functions and Hash Codes
Handling Collisions Using Open Addressing
Handling Collisions Using Separate Chaining
Load Factor and Rehashing
Implementing a Map Using Hashing
Implementing Set Using Hashing
Chapter 28 Graphs and Applications
28.1
28.2
28.3
28.4
28.5
28.6
28.7
28.8
28.9
28.10
Introduction
Basic Graph Terminologies
Representing Graphs
Modeling Graphs
Graph Visualization
Graph Traversals
Depth-First Search (DFS)
Case Study: The Connected Circles Problem
Breadth-First Search (BFS)
Case Study: The Nine Tails Problem
Chapter 29 Weighted Graphs and Applications
29.1
29.2
29.3
29.4
29.5
29.6
Introduction
Representing Weighted Graphs
The WeightedGraph Class
Minimum Spanning Trees
Finding Shortest Paths
Case Study: The Weighted Nine Tails Problem
Chapter 30 Multithreading and Parallel
Programming
30.1
30.2
30.3
30.4
30.5
30.6
30.7
30.8
30.9
30.10
30.11
30.12
30.13
30.14
30.15
30.16
Introduction
Thread Concepts
Creating Tasks and Threads
The Thread Class
Case Study: Flashing Text
Thread Pools
Thread Synchronization
Synchronization Using Locks
Cooperation among Threads
Case Study: Producer/Consumer
Blocking Queues
Semaphores
Avoiding Deadlocks
Thread States
Synchronized Collections
Parallel Programming
970
971
972
972
978
981
985
986
986
987
989
993
993
995
1004
1015
1016
1017
1019
1024
1034
1037
1038
1042
1045
1048
1061
1062
1063
1065
1072
1078
1086
1097
1098
1098
1098
1102
1105
1106
1108
1112
1114
1119
1122
1124
1126
1126
1127
1128
Contents xix
Chapter 31 Networking
1139
Introduction
Client/Server Computing
The InetAddress Class
Serving Multiple Clients
Sending and Receiving Objects
Case Study: Distributed Tic-Tac-Toe Games
1140
1140
1147
1148
1151
1156
Chapter 32 Java Database Programming
1173
31.1
31.2
31.3
31.4
31.5
31.6
32.1
32.2
32.3
32.4
32.5
32.6
32.7
Introduction
Relational Database Systems
SQL
JDBC
PreparedStatement
CallableStatement
Retrieving Metadata
Chapter 33 JavaServer Faces
33.1
33.2
33.3
33.4
33.5
33.6
33.7
33.8
33.9
Introduction
Getting Started with JSF
JSF GUI Components
Processing the Form
Case Study: Calculator
Session Tracking
Validating Input
Binding Database with Facelets
Opening New JSF Pages
1174
1174
1178
1189
1197
1199
1202
1213
1214
1214
1222
1226
1230
1233
1235
1239
1245
Bonus Chapters 34–42 are available from the Companion Website at
www.pearsonhighered.com/liang:
Chapter 34 Advanced JavaFX
34-1
Chapter 35 Advanced Database Programming
35-1
Chapter 36 Internationalization
36-1
Chapter 37 Servlets
37-1
Chapter 38 JavaServer Pages
38-1
Chapter 39 Web Services
39-1
Chapter 40 2-4 Trees and B-Trees
40-1
Chapter 41 Red-Black Trees
41-1
Chapter 42 Testing Using JUnit
42-1
xx Contents
APPENDIXES
Appendix A
Appendix B
Appendix C
Appendix D
Appendix E
Appendix F
Appendix G
Appendix H
Appendix I
INDEX
Java Keywords
1263
The ASCII Character Set
1266
Operator Precedence Chart
1268
Java Modifiers
1270
Special Floating-Point Values
1272
Number Systems
1273
Bitwise Operations
1277
Regular Expressions
1278
Enumerated Types
1283
1289
VideoNotes
VideoNote
Locations of VideoNotes
http://www.pearsonhighered.com/liang
Chapter 1
Chapter 2
Chapter 3
Chapter 4
Chapter 5
Chapter 6
Chapter 7
Introduction to Computers, Programs,
and Java
1
Your first Java program
Compile and run a Java program
NetBeans brief tutorial
Eclipse brief tutorial
12
17
23
25
Elementary Programming
33
Obtain input
Use operators / and %
Software development process
Compute loan payments
Compute BMI
37
52
59
60
72
Selections
75
Program addition quiz
Program subtraction quiz
Use multi-way if-else statements
Sort three integers
Check point location
77
87
90
110
112
Mathematical Functions,
Characters, and Strings
119
Introduce math functions
Introduce strings and objects
Convert hex to decimal
Compute great circle distance
Convert hex to binary
120
130
143
151
153
Loops
157
Guess a number
Multiple subtraction quiz
Minimize numeric errors
Display loan schedule
Sum a series
161
164
178
194
195
Methods
203
Define/invoke max method
Use void method
Modularize code
Stepwise refinement
Reverse an integer
Estimate p
206
209
215
225
234
237
Single-Dimensional Arrays
245
Random shuffling
Deck of cards
Selection sort
250
254
269
Chapter 8
Chapter 9
Command-line arguments
Coupon collector’s problem
Consecutive four
272
281
283
Multidimensional Arrays
287
Find the row with the largest sum
Grade multiple-choice test
Sudoku
Multiply two matrices
Even number of 1s
292
294
298
307
314
Objects and Classes
321
Define classes and objects
Use classes
Static vs. instance
Data field encapsulation
The Fan class
322
334
337
344
362
Chapter 10 Object-Oriented Thinking
The Loan class
The BMI class
The StackOfIntegers class
Process large numbers
The String class
The MyPoint class
Chapter 11 Inheritance and Polymorphism
Geometric class hierarchy
Polymorphism and dynamic binding demo
The ArrayList class
The MyStack class
New Account class
Chapter 12 Exception Handling and Text I/O
Exception-handling advantages
Create custom exception classes
Write and read data
HexFormatException
Chapter 13 Abstract Classes and Interfaces
Abstract GeometricObject class
Calendar and GregorianCalendar classes
The concept of interface
Redesign the Rectangle class
Chapter 14 JavaFX Basics
Understand property binding
Use Image and ImageView
Use layout panes
365
367
370
378
384
386
400
409
410
424
432
439
446
449
450
470
476
489
495
496
503
506
530
535
542
549
552
xxi
xxii VideoNotes
Use shapes
Display a tictactoe board
Display a bar chart
Chapter 15 Event-Driven Programming
and Animations
Handler and its registration
Anonymous handler
Move message using the mouse
Animate a rising flag
Flashing text
Simple calculator
Check mouse point location
Display a running fan
Chapter 16 JavaFX UI Controls and Multimedia
Use ListView
Use Slider
560
578
580
TicTacToe
Use Media, MediaPlayer, and MediaView
Audio and image
Use radio buttons and text fields
Set fonts
657
662
666
669
671
585
592
595
602
608
614
621
622
625
629
647
654
Chapter 17 Binary I/O
Copy file
Object I/O
Split a large file
Chapter 18 Recursion
Binary search
Directory size
Fractal (Sierpinski triangle)
Search a string in a directory
Recursive tree
677
691
693
702
705
716
717
722
733
736
CHAPTER
1
INTRODUCTION
TO COMPUTERS,
PROGRAMS,
AND JAVA
Objectives
■
To understand computer basics, programs, and operating systems
(§§1.2–1.4).
■
To describe the relationship between Java and the World Wide Web
(§1.5).
■
To understand the meaning of Java language specification, API, JDK,
and IDE (§1.6).
■
To write a simple Java program (§1.7).
■
To display output on the console (§1.7).
■
To explain the basic syntax of a Java program (§1.7).
■
To create, compile, and run Java programs (§1.8).
■
To use sound Java programming style and document programs properly
(§1.9).
■
To explain the differences between syntax errors, runtime errors, and
logic errors (§1.10).
■
To develop Java programs using NetBeans (§1.11).
■
To develop Java programs using Eclipse (§1.12).
2 Chapter 1
Introduction to Computers, Programs, and Java
1.1 Introduction
Key
Point
what is programming?
programming
program
The central theme of this book is to learn how to solve problems by writing a program.
This book is about programming. So, what is programming? The term programming means to
create (or develop) software, which is also called a program. In basic terms, software contains
the instructions that tell a computer—or a computerized device—what to do.
Software is all around you, even in devices that you might not think would need it. Of
course, you expect to find and use software on a personal computer, but software also plays a
role in running airplanes, cars, cell phones, and even toasters. On a personal computer, you use
word processors to write documents, Web browsers to explore the Internet, and e-mail programs to send and receive messages. These programs are all examples of software. Software
developers create software with the help of powerful tools called programming languages.
This book teaches you how to create programs by using the Java programming language.
There are many programming languages, some of which are decades old. Each language
was invented for a specific purpose—to build on the strengths of a previous language, for
example, or to give the programmer a new and unique set of tools. Knowing that there are
so many programming languages available, it would be natural for you to wonder which
one is best. But, in truth, there is no “best” language. Each one has its own strengths and
weaknesses. Experienced programmers know that one language might work well in some
situations, whereas a different language may be more appropriate in other situations. For this
reason, seasoned programmers try to master as many different programming languages as
they can, giving them access to a vast arsenal of software-development tools.
If you learn to program using one language, you should find it easy to pick up other languages.
The key is to learn how to solve problems using a programming approach. That is the main
theme of this book.
You are about to begin an exciting journey: learning how to program. At the outset, it is
helpful to review computer basics, programs, and operating systems. If you are already familiar
with such terms as CPU, memory, disks, operating systems, and programming languages, you
may skip Sections 1.2–1.4.
1.2 What Is a Computer?
Key
Point
hardware
software
bus
A computer is an electronic device that stores and processes data.
A computer includes both hardware and software. In general, hardware comprises the visible,
physical elements of the computer, and software provides the invisible instructions that control
the hardware and make it perform specific tasks. Knowing computer hardware isn’t essential
to learning a programming language, but it can help you better understand the effects that
a program’s instructions have on the computer and its components. This section introduces
computer hardware components and their functions.
A computer consists of the following major hardware components (Figure 1.1):
■
A central processing unit (CPU)
■
Memory (main memory)
■
Storage devices (such as disks and CDs)
■
Input devices (such as the mouse and keyboard)
■
Output devices (such as monitors and printers)
■
Communication devices (such as modems and network interface cards)
A computer’s components are interconnected by a subsystem called a bus. You can think
of a bus as a sort of system of roads running among the computer’s components; data and
power travel along the bus from one part of the computer to another. In personal computers,
1.2 What Is a Computer? 3
Bus
Storage
Devices
Memory
CPU
e.g., Disk, CD,
and Tape
Communication
Devices
Input
Devices
Output
Devices
e.g., Modem
and NIC
e.g., Keyboard,
Mouse
e.g., Monitor,
Printer
FIGURE 1.1 A computer consists of a CPU, memory, storage devices, input devices, output
devices, and communication devices.
the bus is built into the computer’s motherboard, which is a circuit case that connects all of
the parts of a computer together.
1.2.1
Central Processing Unit
The central processing unit (CPU) is the computer’s brain. It retrieves instructions from
memory and executes them. The CPU usually has two components: a control unit and an
arithmetic/logic unit. The control unit controls and coordinates the actions of the other
components. The arithmetic/logic unit performs numeric operations (addition, subtraction,
multiplication, division) and logical operations (comparisons).
Today’s CPUs are built on small silicon semiconductor chips that contain millions of tiny
electric switches, called transistors, for processing information.
Every computer has an internal clock, which emits electronic pulses at a constant rate.
These pulses are used to control and synchronize the pace of operations. A higher clock speed
enables more instructions to be executed in a given period of time. The unit of measurement of
clock speed is the hertz (Hz), with 1 hertz equaling 1 pulse per second. In the 1990s, computers
measured clocked speed in megahertz (MHz), but CPU speed has been improving continuously;
the clock speed of a computer is now usually stated in gigahertz (GHz). Intel’s newest processors run at about 3 GHz.
CPUs were originally developed with only one core. The core is the part of the processor
that performs the reading and executing of instructions. In order to increase CPU processing
power, chip manufacturers are now producing CPUs that contain multiple cores. A multicore
CPU is a single component with two or more independent cores. Today’s consumer computers typically have two, three, and even four separate cores. Soon, CPUs with dozens or even
hundreds of cores will be affordable.
1.2.2
motherboard
CPU
speed
hertz
megahertz
gigahertz
core
Bits and Bytes
Before we discuss memory, let’s look at how information (data and programs) are stored in
a computer.
A computer is really nothing more than a series of switches. Each switch exists in two
states: on or off. Storing information in a computer is simply a matter of setting a sequence of
switches on or off. If the switch is on, its value is 1. If the switch is off, its value is 0. These 0s
and 1s are interpreted as digits in the binary number system and are called bits (binary digits).
The minimum storage unit in a computer is a byte. A byte is composed of eight bits. A
small number such as 3 can be stored as a single byte. To store a number that cannot fit into a
single byte, the computer uses several bytes.
Data of various kinds, such as numbers and characters, are encoded as a series of bytes.
As a programmer, you don’t need to worry about the encoding and decoding of data, which
the computer system performs automatically, based on the encoding scheme. An encoding
scheme is a set of rules that govern how a computer translates characters, numbers, and symbols into data the computer can actually work with. Most schemes translate each character
bits
byte
encoding scheme
4 Chapter 1
Introduction to Computers, Programs, and Java
into a predetermined string of bits. In the popular ASCII encoding scheme, for example, the
character C is represented as 01000011 in one byte.
A computer’s storage capacity is measured in bytes and multiples of the byte, as follows:
kilobyte (KB)
■
A kilobyte (KB) is about 1,000 bytes.
megabyte (MB)
■
A megabyte (MB) is about 1 million bytes.
gigabyte (GB)
■
A gigabyte (GB) is about 1 billion bytes.
terabyte (TB)
■
A terabyte (TB) is about 1 trillion bytes.
A typical one-page word document might take 20 KB. Therefore, 1 MB can store 50 pages
of documents and 1 GB can store 50,000 pages of documents. A typical two-hour highresolution movie might take 8 GB, so it would require 160 GB to store 20 movies.
1.2.3
memory
unique address
RAM
Memory
A computer’s memory consists of an ordered sequence of bytes for storing programs as well as
data that the program is working with. You can think of memory as the computer’s work area
for executing a program. A program and its data must be moved into the computer’s memory
before they can be executed by the CPU.
Every byte in the memory has a unique address, as shown in Figure 1.2. The address is
used to locate the byte for storing and retrieving the data. Since the bytes in the memory can
be accessed in any order, the memory is also referred to as random-access memory (RAM).
Memory address
2000
2001
2002
2003
2004
FIGURE 1.2
locations.
Memory content
01000011
01110010
01100101
01110111
00000011
Encoding for character ‘C’
Encoding for character ‘r’
Encoding for character ‘e’
Encoding for character ‘w’
Encoding for number 3
Memory stores data and program instructions in uniquely addressed memory
Today’s personal computers usually have at least 4 gigabyte of RAM, but they more commonly have 6 to 8 GB installed. Generally speaking, the more RAM a computer has, the faster
it can operate, but there are limits to this simple rule of thumb.
A memory byte is never empty, but its initial content may be meaningless to your program.
The current content of a memory byte is lost whenever new information is placed in it.
Like the CPU, memory is built on silicon semiconductor chips that have millions of transistors embedded on their surface. Compared to CPU chips, memory chips are less complicated,
slower, and less expensive.
1.2.4
storage devices
Storage Devices
A computer’s memory (RAM) is a volatile form of data storage: any information that has
been stored in memory (i.e., saved) is lost when the system’s power is turned off. Programs
and data are permanently stored on storage devices and are moved, when the computer
1.2 What Is a Computer? 5
actually uses them, to memory, which operates at much faster speeds than permanent storage
devices can.
There are three main types of storage devices:
■
Magnetic disk drives
■
Optical disc drives (CD and DVD)
■
USB flash drives
Drives are devices for operating a medium, such as disks and CDs. A storage medium
physically stores data and program instructions. The drive reads data from the medium and
writes data onto the medium.
drive
Disks
A computer usually has at least one hard disk drive. Hard disks are used for permanently storing data and programs. Newer computers have hard disks that can store from 500 gigabytes to
1 terabytes of data. Hard disk drives are usually encased inside the computer, but removable
hard disks are also available.
hard disk
CDs and DVDs
CD stands for compact disc. There are two types of CD drives: CD-R and CD-RW. A CD-R is
for read-only permanent storage; the user cannot modify its contents once they are recorded.
A CD-RW can be used like a hard disk; that is, you can write data onto the disc, and then
overwrite that data with new data. A single CD can hold up to 700 MB. Most new PCs are
equipped with a CD-RW drive that can work with both CD-R and CD-RW discs.
DVD stands for digital versatile disc or digital video disc. DVDs and CDs look alike, and
you can use either to store data. A DVD can hold more information than a CD; a standard
DVD’s storage capacity is 4.7 GB. Like CDs, there are two types of DVDs: DVD-R (readonly) and DVD-RW (rewritable).
CD-R
CD-RW
DVD
USB Flash Drives
Universal serial bus (USB) connectors allow the user to attach many kinds of peripheral
devices to the computer. You can use a USB to connect a printer, digital camera, mouse,
external hard disk drive, and other devices to the computer.
A USB flash drive is a device for storing and transporting data. A flash drive is small—
about the size of a pack of gum. It acts like a portable hard drive that can be plugged into your
computer’s USB port. USB flash drives are currently available with up to 256 GB storage
capacity.
1.2.5
Input and Output Devices
Input and output devices let the user communicate with the computer. The most common input
devices are keyboards and mice. The most common output devices are monitors and printers.
The Keyboard
A keyboard is a device for entering input. Compact keyboards are available without a numeric
keypad.
Function keys are located across the top of the keyboard and are prefaced with the letter F.
Their functions depend on the software currently being used.
A modifier key is a special key (such as the Shift, Alt, and Ctrl keys) that modifies the normal action of another key when the two are pressed simultaneously.
The numeric keypad, located on the right side of most keyboards, is a separate set of keys
styled like a calculator to use for entering numbers quickly.
Arrow keys, located between the main keypad and the numeric keypad, are used to move
the mouse pointer up, down, left, and right on the screen in many kinds of programs.
function key
modifier key
numeric keypad
arrow keys
6 Chapter 1
Introduction to Computers, Programs, and Java
The Insert, Delete, Page Up, and Page Down keys are used in word processing and other
programs for inserting text and objects, deleting text and objects, and moving up or down
through a document one screen at a time.
Insert key
Delete key
Page Up key
Page Down key
The Mouse
A mouse is a pointing device. It is used to move a graphical pointer (usually in the shape of
an arrow) called a cursor around the screen or to click on-screen objects (such as a button) to
trigger them to perform an action.
The Monitor
The monitor displays information (text and graphics). The screen resolution and dot pitch
determine the quality of the display.
The screen resolution specifies the number of pixels in horizontal and vertical dimensions
of the display device. Pixels (short for “picture elements”) are tiny dots that form an image on
the screen. A common resolution for a 17-inch screen, for example, is 1,024 pixels wide and
768 pixels high. The resolution can be set manually. The higher the resolution, the sharper
and clearer the image is.
The dot pitch is the amount of space between pixels, measured in millimeters. The smaller
the dot pitch, the sharper the display.
screen resolution
pixels
dot pitch
1.2.6
Communication Devices
Computers can be networked through communication devices, such as a dial-up modem
(modulator/demodulator), a DSL or cable modem, a wired network interface card, or a wireless adapter.
dial-up modem
■
A dial-up modem uses a phone line and can transfer data at a speed up to 56,000 bps
(bits per second).
digital subscriber line (DSL)
■
A digital subscriber line (DSL) connection also uses a standard phone line, but it can
transfer data 20 times faster than a standard dial-up modem.
cable modem
■
A cable modem uses the cable TV line maintained by the cable company and is generally faster than DSL.
network interface card (NIC)
local area network (LAN)
million bits per second
(mbps)
■
A network interface card (NIC) is a device that connects a computer to a local area
network (LAN). LANs are commonly used in universities, businesses, and government
agencies. A high-speed NIC called 1000BaseT can transfer data at 1,000 million bits
per second (mbps).
■
Wireless networking is now extremely popular in homes, businesses, and schools.
Every laptop computer sold today is equipped with a wireless adapter that enables
the computer to connect to a local area network and the Internet.
Note
Answers to checkpoint questions are on the Companion Website.
✓
Check
Point
1.1
1.2
1.3
1.4
1.5
1.6
1.7
What are hardware and software?
List five major hardware components of a computer.
What does the acronym “CPU” stand for?
What unit is used to measure CPU speed?
What is a bit? What is a byte?
What is memory for? What does RAM stand for? Why is memory called RAM?
What unit is used to measure memory size?
1.3 Programming Languages 7
1.8
1.9
What unit is used to measure disk size?
What is the primary difference between memory and a storage device?
1.3 Programming Languages
Computer programs, known as software, are instructions that tell a computer what to do.
Computers do not understand human languages, so programs must be written in a language a
computer can use. There are hundreds of programming languages, and they were developed
to make the programming process easier for people. However, all programs must be converted
into the instructions the computer can execute.
1.3.1
Key
Point
Machine Language
A computer’s native language, which differs among different types of computers, is its
machine language—a set of built-in primitive instructions. These instructions are in the form
of binary code, so if you want to give a computer an instruction in its native language, you
have to enter the instruction as binary code. For example, to add two numbers, you might have
to write an instruction in binary code, like this:
machine language
1101101010011010
1.3.2
Assembly Language
Programming in machine language is a tedious process. Moreover, programs written in
machine language are very difficult to read and modify. For this reason, assembly language
was created in the early days of computing as an alternative to machine languages. Assembly
language uses a short descriptive word, known as a mnemonic, to represent each of the
machine-language instructions. For example, the mnemonic add typically means to add numbers and sub means to subtract numbers. To add the numbers 2 and 3 and get the result, you
might write an instruction in assembly code like this:
assembly language
add 2, 3, result
Assembly languages were developed to make programming easier. However, because the
computer cannot execute assembly language, another program—called an assembler—is used
to translate assembly-language programs into machine code, as shown in Figure 1.3.
Assembly Source File
...
add 2, 3, result
...
assembler
Machine-Code File
Assembler
...
1101101010011010
...
FIGURE 1.3 An assembler translates assembly-language instructions into machine code.
Writing code in assembly language is easier than in machine language. However, it is
still tedious to write code in assembly language. An instruction in assembly language essentially corresponds to an instruction in machine code. Writing in assembly requires that you
know how the CPU works. Assembly language is referred to as a low-level language, because
assembly language is close in nature to machine language and is machine dependent.
low-level language
8 Chapter 1
Introduction to Computers, Programs, and Java
1.3.3
High-Level Language
In the 1950s, a new generation of programming languages known as high-level languages
emerged. They are platform independent, which means that you can write a program in a highlevel language and run it in different types of machines. High-level languages are English-like
and easy to learn and use. The instructions in a high-level programming language are called
statements. Here, for example, is a high-level language statement that computes the area of a
circle with a radius of 5:
high-level language
statement
area = 5 * 5 * 3.14159;
There are many high-level programming languages, and each was designed for a specific
purpose. Table 1.1 lists some popular ones.
TABLE 1.1 Popular High-Level Programming Languages
Language
Description
Ada
Named for Ada Lovelace, who worked on mechanical general-purpose computers. The Ada language was
developed for the Department of Defense and is used mainly in defense projects.
BASIC
Beginner’s All-purpose Symbolic Instruction Code. It was designed to be learned and used easily by beginners.
C
Developed at Bell Laboratories. C combines the power of an assembly language with the ease of use and
portability of a high-level language.
C++
C++ is an object-oriented language, based on C.
C#
Pronounced “C Sharp.” It is a hybrid of Java and C++ and was developed by Microsoft.
COBOL
COmmon Business Oriented Language. Used for business applications.
FORTRAN
FORmula TRANslation. Popular for scientific and mathematical applications.
Java
Developed by Sun Microsystems, now part of Oracle. It is widely used for developing platform-independent
Internet applications.
Pascal
Named for Blaise Pascal, who pioneered calculating machines in the seventeenth century. It is a simple,
structured, general-purpose language primarily for teaching programming.
Python
A simple general-purpose scripting language good for writing short programs.
Visual Basic
Visual Basic was developed by Microsoft and it enables the programmers to rapidly develop graphical user
interfaces.
A program written in a high-level language is called a source program or source code.
Because a computer cannot execute a source program, a source program must be translated
into machine code for execution. The translation can be done using another programming tool
called an interpreter or a compiler.
source program
source code
interpreter
compiler
✓
Check
Point
1.10
1.11
1.12
1.13
1.14
■
An interpreter reads one statement from the source code, translates it to the machine code
or virtual machine code, and then executes it right away, as shown in Figure 1.4a. Note
that a statement from the source code may be translated into several machine instructions.
■
A compiler translates the entire source code into a machine-code file, and the
machine-code file is then executed, as shown in Figure 1.4b.
What language does the CPU understand?
What is an assembly language?
What is an assembler?
What is a high-level programming language?
What is a source program?
1.4 Operating Systems 9
1.15
1.16
1.17
What is an interpreter?
What is a compiler?
What is the difference between an interpreted language and a compiled language?
High-Level Source File
Output
...
area = 5 * 5 * 3.1415;
...
Interpreter
(a)
High-Level Source File
...
area = 5 * 5 * 3.1415;
...
Machine-Code File
Compiler
...
0101100011011100
1111100011000100
...
Output
Executor
(b)
FIGURE 1.4 (a) An interpreter translates and executes a program one statement at a time. (b) A compiler translates the
entire source program into a machine-language file for execution.
1.4 Operating Systems
The operating system (OS) is the most important program that runs on a computer.
The OS manages and controls a computer’s activities.
The popular operating systems for general-purpose computers are Microsoft Windows, Mac
OS, and Linux. Application programs, such as a Web browser or a word processor, cannot
run unless an operating system is installed and running on the computer. Figure 1.5 shows the
interrelationship of hardware, operating system, application software, and the user.
User
Application Programs
Operating System
Hardware
FIGURE 1.5 Users and applications access the computer’s hardware via the operating system.
The major tasks of an operating system are as follows:
■
Controlling and monitoring system activities
■
Allocating and assigning system resources
■
Scheduling operations
Key
Point
operating system (OS)
10 Chapter 1
Introduction to Computers, Programs, and Java
1.4.1
Controlling and Monitoring System Activities
Operating systems perform basic tasks, such as recognizing input from the keyboard, sending
output to the monitor, keeping track of files and folders on storage devices, and controlling
peripheral devices, such as disk drives and printers. An operating system must also ensure
that different programs and users working at the same time do not interfere with each other.
In addition, the OS is responsible for security, ensuring that unauthorized users and programs
are not allowed to access the system.
1.4.2
Allocating and Assigning System Resources
The operating system is responsible for determining what computer resources a program
needs (such as CPU time, memory space, disks, input and output devices) and for allocating
and assigning them to run the program.
1.4.3
Scheduling Operations
The OS is responsible for scheduling programs’ activities to make efficient use of system
resources. Many of today’s operating systems support techniques such as multiprogramming,
multithreading, and multiprocessing to increase system performance.
Multiprogramming allows multiple programs to run simultaneously by sharing the same
CPU. The CPU is much faster than the computer’s other components. As a result, it is idle
most of the time—for example, while waiting for data to be transferred from a disk or waiting
for other system resources to respond. A multiprogramming OS takes advantage of this
situation by allowing multiple programs to use the CPU when it would otherwise be idle. For
example, multiprogramming enables you to use a word processor to edit a file at the same time
as your Web browser is downloading a file.
Multithreading allows a single program to execute multiple tasks at the same time. For
instance, a word-processing program allows users to simultaneously edit text and save it to a
disk. In this example, editing and saving are two tasks within the same application. These two
tasks may run concurrently.
Multiprocessing, or parallel processing, uses two or more processors together to perform subtasks concurrently and then combine solutions of the subtasks to obtain a solution
for the entire task. It is like a surgical operation where several doctors work together on
one patient.
multiprogramming
multithreading
multiprocessing
✓
Check
Point
1.18
1.19
1.20
What is an operating system? List some popular operating systems.
What are the major responsibilities of an operating system?
What are multiprogramming, multithreading, and multiprocessing?
1.5 Java, the World Wide Web, and Beyond
Key
Point
Java is a powerful and versatile programming language for developing software
running on mobile devices, desktop computers, and servers.
This book introduces Java programming. Java was developed by a team led by James Gosling
at Sun Microsystems. Sun Microsystems was purchased by Oracle in 2010. Originally called
Oak, Java was designed in 1991 for use in embedded chips in consumer electronic appliances.
In 1995, renamed Java, it was redesigned for developing Web applications. For the history of
Java, see www.java.com/en/javahistory/index.jsp.
Java has become enormously popular. Its rapid rise and wide acceptance can be traced
to its design characteristics, particularly its promise that you can write a program once
and run it anywhere. As stated by its designer, Java is simple, object oriented, distributed,
1.6 The Java Language Specification, API, JDK, and IDE 11
interpreted, robust, secure, architecture neutral, portable, high performance, multithreaded, and dynamic. For the anatomy of Java characteristics, see www.cs.armstrong.edu/
liang/JavaCharacteristics.pdf.
Java is a full-featured, general-purpose programming language that can be used to develop
robust mission-critical applications. Today, it is employed not only for Web programming but
also for developing standalone applications across platforms on servers, desktop computers,
and mobile devices. It was used to develop the code to communicate with and control the
robotic rover on Mars. Many companies that once considered Java to be more hype than substance are now using it to create distributed applications accessed by customers and partners
across the Internet. For every new project being developed today, companies are asking how
they can use Java to make their work easier.
The World Wide Web is an electronic information repository that can be accessed on the
Internet from anywhere in the world. The Internet, the Web’s infrastructure, has been around
for more than forty years. The colorful World Wide Web and sophisticated Web browsers are
the major reason for the Internet’s popularity.
Java initially became attractive because Java programs can be run from a Web browser.
Such programs are called applets. Applets employ a modern graphical interface with buttons, text fields, text areas, radio buttons, and so on, to interact with users on the Web and
process their requests. Applets make the Web responsive, interactive, and fun to use. Applets
are embedded in an HTML file. HTML (Hypertext Markup Language) is a simple scripting
language for laying out documents, linking documents on the Internet, and bringing images,
sound, and video alive on the Web. Today, you can use Java to develop rich Internet applications. A rich Internet application (RIA) is a Web application designed to deliver the same
features and functions normally associated with deskop applications.
Java is now very popular for developing applications on Web servers. These applications
process data, perform computations, and generate dynamic Web pages. Many commercial
Websites are developed using Java on the backend.
Java is a versatile programming language: you can use it to develop applications for desktop computers, servers, and small handheld devices. The software for Android cell phones is
developed using Java.
1.21
1.22
1.23
Who invented Java? Which company owns Java now?
What is a Java applet?
What programming language does Android use?
✓
Check
Point
1.6 The Java Language Specification, API, JDK, and IDE
Java syntax is defined in the Java language specification, and the Java library is
defined in the Java API. The JDK is the software for developing and running Java
programs. An IDE is an integrated development environment for rapidly developing
programs.
Computer languages have strict rules of usage. If you do not follow the rules when writing a
program, the computer will not be able to understand it. The Java language specification and
the Java API define the Java standards.
The Java language specification is a technical definition of the Java programming
language’s syntax and semantics. You can find the complete Java language specification at
http://docs.oracle.com/javase/specs/.
The application program interface (API), also known as library, contains predefined
classes and interfaces for developing Java programs. The API is still expanding. You can
view and download the latest version of the Java API at http://download.java.net/jdk8/docs/api/.
Key
Point
Java language specification
API
library
12 Chapter 1
Introduction to Computers, Programs, and Java
Java is a full-fledged and powerful language that can be used in many ways. It comes in
three editions:
Java SE, EE, and ME
■
Java Standard Edition (Java SE) to develop client-side applications. The applications can run standalone or as applets running from a Web browser.
■
Java Enterprise Edition (Java EE) to develop server-side applications, such as Java
servlets, JavaServer Pages (JSP), and JavaServer Faces (JSF).
■
Java Micro Edition (Java ME) to develop applications for mobile devices, such as
cell phones.
This book uses Java SE to introduce Java programming. Java SE is the foundation upon
which all other Java technology is based. There are many versions of Java SE. The latest,
Java SE 8, is used in this book. Oracle releases each version with a Java Development Toolkit
(JDK). For Java SE 8, the Java Development Toolkit is called JDK 1.8 (also known as Java 8
or JDK 8).
The JDK consists of a set of separate programs, each invoked from a command line, for
developing and testing Java programs. Instead of using the JDK, you can use a Java development tool (e.g., NetBeans, Eclipse, and TextPad)—software that provides an integrated
development environment (IDE) for developing Java programs quickly. Editing, compiling,
building, debugging, and online help are integrated in one graphical user interface. You simply
enter source code in one window or open an existing file in a window, and then click a button
or menu item or press a function key to compile and run the program.
Java Development
Toolkit (JDK)
JDK 1.8 = JDK 8
Integrated development
environment
✓
Check
Point
1.24
1.25
1.26
1.27
What is the Java language specification?
What does JDK stand for?
What does IDE stand for?
Are tools like NetBeans and Eclipse different languages from Java, or are they
dialects or extensions of Java?
1.7 A Simple Java Program
Key
Point
what is a console?
console input
console output
A Java program is executed from the main method in the class.
Let’s begin with a simple Java program that displays the message Welcome to Java! on the
console. (The word console is an old computer term that refers to the text entry and display
device of a computer. Console input means to receive input from the keyboard, and console
output means to display output on the monitor.) The program is shown in Listing 1.1.
LISTING 1.1 Welcome.java
class
main method
display message
VideoNote
1
2
3
4
5
6
public class Welcome {
public static void main(String[] args) {
// Display message Welcome to Java! on the console
System.out.println("Welcome to Java!");
}
}
Your first Java program
Welcome to Java!
line numbers
Note that the line numbers are for reference purposes only; they are not part of the program.
So, don’t type line numbers in your program.
1.7 A Simple Java Program 13
Line 1 defines a class. Every Java program must have at least one class. Each class has a
name. By convention, class names start with an uppercase letter. In this example, the class
name is Welcome.
Line 2 defines the main method. The program is executed from the main method. A class
may contain several methods. The main method is the entry point where the program begins
execution.
A method is a construct that contains statements. The main method in this program contains the System.out.println statement. This statement displays the string Welcome to
Java! on the console (line 4). String is a programming term meaning a sequence of characters. A string must be enclosed in double quotation marks. Every statement in Java ends with
a semicolon (;), known as the statement terminator.
Reserved words, or keywords, have a specific meaning to the compiler and cannot be used
for other purposes in the program. For example, when the compiler sees the word class, it
understands that the word after class is the name for the class. Other reserved words in this
program are public, static, and void.
Line 3 is a comment that documents what the program is and how it is constructed. Comments
help programmers to communicate and understand the program. They are not programming
statements and thus are ignored by the compiler. In Java, comments are preceded by two
slashes (//) on a line, called a line comment, or enclosed between /* and */ on one or several
lines, called a block comment or paragraph comment. When the compiler sees //, it ignores
all text after // on the same line. When it sees /*, it scans for the next */ and ignores any text
between /* and */. Here are examples of comments:
class name
main method
string
statement terminator
reserved word
keyword
comment
line comment
block comment
// This application program displays Welcome to Java!
/* This application program displays Welcome to Java! */
/* This application program
displays Welcome to Java! */
A pair of curly braces in a program forms a block that groups the program’s components.
In Java, each block begins with an opening brace ({) and ends with a closing brace (}). Every
class has a class block that groups the data and methods of the class. Similarly, every method
has a method block that groups the statements in the method. Blocks can be nested, meaning
that one block can be placed within another, as shown in the following code.
block
public class Welcome {
public static void main(String[] args) {
Class block
System.out.println("Welcome to Java!"); Method block
}
}
Tip
An opening brace must be matched by a closing brace. Whenever you type an opening
brace, immediately type a closing brace to prevent the missing-brace error. Most Java
IDEs automatically insert the closing brace for each opening brace.
match braces
Caution
Java source programs are case sensitive. It would be wrong, for example, to replace
main in the program with Main.
You have seen several special characters (e.g., { }, //, ;) in the program. They are used
in almost every program. Table 1.2 summarizes their uses.
The most common errors you will make as you learn to program will be syntax errors.
Like any programming language, Java has its own syntax, and you need to write code that
case sensitive
special characters
common errors
14 Chapter 1
Introduction to Computers, Programs, and Java
TABLE 1.2 Special Characters
syntax rules
Character
Name
Description
{}
Opening and closing braces
Denote a block to enclose statements.
()
Opening and closing parentheses
Used with methods.
[]
Opening and closing brackets
Denote an array.
//
Double slashes
Precede a comment line.
""
Opening and closing quotation marks
Enclose a string (i.e., sequence of characters).
;
Semicolon
Mark the end of a statement.
conforms to the syntax rules. If your program violates a rule—for example, if the semicolon
is missing, a brace is missing, a quotation mark is missing, or a word is misspelled—the Java
compiler will report syntax errors. Try to compile the program with these errors and see what
the compiler reports.
Note
You are probably wondering why the main method is defined this way and why
System.out.println(...) is used to display a message on the console. For the
time being, simply accept that this is how things are done. Your questions will be fully
answered in subsequent chapters.
The program in Listing 1.1 displays one message. Once you understand the program, it
is easy to extend it to display more messages. For example, you can rewrite the program to
display three messages, as shown in Listing 1.2.
LISTING 1.2 WelcomeWithThreeMessages.java
class
main method
display message
1
2
3
4
5
6
7
public class WelcomeWithThreeMessages {
public static void main(String[] args) {
System.out.println("Programming is fun!");
System.out.println("Fundamentals First");
System.out.println("Problem Driven");
}
}
Programming is fun!
Fundamentals First
Problem Driven
Further, you can perform mathematical computations and display the result on the console.
10.5 + 2 * 3
Listing 1.3 gives an example of evaluating
.
45 - 3.5
LISTING 1.3 ComputeExpression.java
class
main method
compute expression
1
2
3
4
5
public class ComputeExpression {
public static void main(String[] args) {
System.out.println((10.5 + 2 * 3) / (45 – 3.5));
}
}
0.39759036144578314
1.8 Creating, Compiling, and Executing a Java Program 15
The multiplication operator in Java is *. As you can see, it is a straightforward process
to translate an arithmetic expression to a Java expression. We will discuss Java expressions
further in Chapter 2.
1.28
1.29
1.30
1.31
1.32
What is a keyword? List some Java keywords.
Is Java case sensitive? What is the case for Java keywords?
What is a comment? Is the comment ignored by the compiler? How do you denote a
comment line and a comment paragraph?
What is the statement to display a string on the console?
Show the output of the following code:
✓
Check
Point
public class Test {
public static void main(String[] args) {
System.out.println("3.5 * 4 / 2 – 2.5 is ");
System.out.println(3.5 * 4 / 2 – 2.5);
}
}
1.8 Creating, Compiling, and Executing a Java Program
You save a Java program in a .java file and compile it into a .class file. The .class file
is executed by the Java Virtual Machine.
You have to create your program and compile it before it can be executed. This process is
repetitive, as shown in Figure 1.6. If your program has compile errors, you have to modify
the program to fix them, and then recompile it. If your program has runtime errors or does not
produce the correct result, you have to modify the program, recompile it, and execute it again.
You can use any text editor or IDE to create and edit a Java source-code file. This section
demonstrates how to create, compile, and run Java programs from a command window.
Sections 1.10 and 1.11 will introduce developing Java programs using NetBeans and Eclipse.
From the command window, you can use a text editor such as Notepad to create the Java
source-code file, as shown in Figure 1.7.
Key
Point
command window
Note
The source file must end with the extension .java and must have the same exact name
as the public class name. For example, the file for the source code in Listing 1.1 should
be named Welcome.java, since the public class name is Welcome.
A Java compiler translates a Java source file into a Java bytecode file. The following command compiles Welcome.java:
file name Welcome.java,
compile
javac Welcome.java
Note
You must first install and configure the JDK before you can compile and run programs.
See Supplement I.B, Installing and Configuring JDK 8, for how to install the JDK and set
up the environment to compile and run Java programs. If you have trouble compiling
and running programs, see Supplement I.C, Compiling and Running Java from the
Command Window. This supplement also explains how to use basic DOS commands
and how to use Windows Notepad to create and edit files. All the supplements are
accessible from the Companion Website at www.cs.armstrong.edu/liang/intro10e/
supplement.html.
If there aren’t any syntax errors, the compiler generates a bytecode file with a .class
extension. Thus, the preceding command generates a file named Welcome.class, as shown
Supplement I.B
Supplement I.C
.class bytecode file
16 Chapter 1
Introduction to Computers, Programs, and Java
Create/Modify Source Code
Source code (developed by the programmer)
Saved on the disk
public class Welcome {
public static void main(String[] args) {
System.out.println("Welcome to Java!");
Source Code
}
}
Bytecode (generated by the compiler for JVM
to read and interpret)
…
Method Welcome()
0 aload_0
…
Compile Source Code
e.g., javac Welcome.java
If compile errors occur
Stored on the disk
Method void main(java.lang.String[])
0 getstatic #2 …
3 ldc #3 <String "Welcome to Java!">
5 invokevirtual #4 …
8 return
Bytecode
Run Bytecode
e.g., java Welcome
“Welcome to Java ” is displayed on the console
Welcome to Java!
Result
If runtime errors or incorrect result
FIGURE 1.6 The Java program-development process consists of repeatedly creating/modifying source code, compiling,
and executing programs.
FIGURE 1.7
bytecode
Java Virtual Machine (JVM)
interpret bytecode
run
You can create a Java source file using Windows Notepad.
in Figure 1.8a. The Java language is a high-level language, but Java bytecode is a low-level
language. The bytecode is similar to machine instructions but is architecture neutral and can
run on any platform that has a Java Virtual Machine (JVM), as shown in Figure 1.8b. Rather
than a physical machine, the virtual machine is a program that interprets Java bytecode. This
is one of Java’s primary advantages: Java bytecode can run on a variety of hardware platforms and operating systems. Java source code is compiled into Java bytecode and Java bytecode is interpreted by the JVM. Your Java code may use the code in the Java library. The JVM
executes your code along with the code in the library.
To execute a Java program is to run the program’s bytecode. You can execute the bytecode
on any platform with a JVM, which is an interpreter. It translates the individual instructions
in the bytecode into the target machine language code one at a time rather than the whole program as a single unit. Each step is executed immediately after it is translated.
The following command runs the bytecode for Listing 1.1:
java Welcome
1.8 Creating, Compiling, and Executing a Java Program 17
Java Bytecode
Java
Compiler
Welcome.class
(Java bytecode
executable file)
Ja
generates
executed
by
JVM
tual Mac
Vir
h
va
e
in
Welcome.java
(Java sourcecode file)
compiled
by
Any
Computer
Library Code
(a)
(b)
FIGURE 1.8 (a) Java source code is translated into bytecode. (b) Java bytecode can be executed on any computer with a
Java Virtual Machine.
Figure 1.9 shows the javac command for compiling Welcome.java. The compiler generates the Welcome.class file, and this file is executed using the java command.
javac command
java command
Note
For simplicity and consistency, all source-code and class files used in this book are
placed under c:\book unless specified otherwise.
c:\book
Compile
Show files
VideoNote
Compile and run a Java
program
Run
FIGURE 1.9
The output of Listing 1.1 displays the message “Welcome to Java!”
Caution
Do not use the extension .class in the command line when executing the program.
Use java ClassName to run the program. If you use java ClassName.class
in the command line, the system will attempt to fetch ClassName.class.class.
java ClassName
Tip
If you execute a class file that does not exist, a NoClassDefFoundError will occur.
If you execute a class file that does not have a main method or you mistype the main
method (e.g., by typing Main instead of main), a NoSuchMethodError will occur.
NoClassDefFoundError
NoSuchMethodError
Note
When executing a Java program, the JVM first loads the bytecode of the class to memory using a program called the class loader. If your program uses other classes, the class
loader dynamically loads them just before they are needed. After a class is loaded, the
JVM uses a program called the bytecode verifier to check the validity of the bytecode and
class loader
bytecode verifier
18 Chapter 1
Introduction to Computers, Programs, and Java
to ensure that the bytecode does not violate Java’s security restrictions. Java enforces
strict security to make sure that Java class files are not tampered with and do not harm
your computer.
Pedagogical Note
Your instructor may require you to use packages for organizing programs. For example,
you may place all programs in this chapter in a package named chapter1. For instructions
on how to use packages, see Supplement I.F, Using Packages to Organize the Classes
in the Text.
use package
✓
Check
Point
1.33
1.34
1.35
1.36
1.37
1.38
1.39
1.40
What is the Java source filename extension, and what is the Java bytecode filename
extension?
What are the input and output of a Java compiler?
What is the command to compile a Java program?
What is the command to run a Java program?
What is the JVM?
Can Java run on any machine? What is needed to run Java on a computer?
If a NoClassDefFoundError occurs when you run a program, what is the cause of
the error?
If a NoSuchMethodError occurs when you run a program, what is the cause of the
error?
1.9 Programming Style and Documentation
Key
Point
programming style
documentation
Good programming style and proper documentation make a program easy to read and
help programmers prevent errors.
Programming style deals with what programs look like. A program can compile and run
properly even if written on only one line, but writing it all on one line would be bad programming style because it would be hard to read. Documentation is the body of explanatory
remarks and comments pertaining to a program. Programming style and documentation are
as important as coding. Good programming style and appropriate documentation reduce the
chance of errors and make programs easy to read. This section gives several guidelines. For
more detailed guidelines, see Supplement I.D, Java Coding Style Guidelines, on the Companion Website.
1.9.1
javadoc comment
Appropriate Comments and Comment Styles
Include a summary at the beginning of the program that explains what the program does, its
key features, and any unique techniques it uses. In a long program, you should also include
comments that introduce each major step and explain anything that is difficult to read. It is
important to make comments concise so that they do not crowd the program or make it difficult to read.
In addition to line comments (beginning with //) and block comments (beginning with
/*), Java supports comments of a special type, referred to as javadoc comments. javadoc comments begin with /** and end with */. They can be extracted into an HTML file using the
JDK’s javadoc command. For more information, see Supplement III.Y, javadoc Comments,
on the companion Website.
Use javadoc comments (/** ... */) for commenting on an entire class or an entire
method. These comments must precede the class or the method header in order to be extracted
into a javadoc HTML file. For commenting on steps inside a method, use line comments (//).
1.9 Programming Style and Documentation 19
To see an example of a javadoc HTML file, check out www.cs.armstrong.edu/liang/javadoc/
Exercise1.html. Its corresponding Java code is shown in www.cs.armstrong.edu/liang/javadoc/
Exercise1.java.
1.9.2
Proper Indentation and Spacing
A consistent indentation style makes programs clear and easy to read, debug, and maintain.
Indentation is used to illustrate the structural relationships between a program’s components
or statements. Java can read the program even if all of the statements are on the same long
line, but humans find it easier to read and maintain code that is aligned properly. Indent each
subcomponent or statement at least two spaces more than the construct within which it is
nested.
A single space should be added on both sides of a binary operator, as shown in the following statement:
1.9.3
System.out.println(3+4*4);
Bad style
System.out.println(3 + 4 * 4);
Good style
indent code
Block Styles
A block is a group of statements surrounded by braces. There are two popular styles, next-line
style and end-of-line style, as shown below.
public class Test
{
public static void main(String[] args)
{
System.out.println("Block Styles");
}
}
public class Test {
public static void main(String[] args) {
System.out.println("Block Styles");
}
}
Next-line style
End-of-line style
The next-line style aligns braces vertically and makes programs easy to read, whereas the
end-of-line style saves space and may help avoid some subtle programming errors. Both are
acceptable block styles. The choice depends on personal or organizational preference. You
should use a block style consistently—mixing styles is not recommended. This book uses the
end-of-line style to be consistent with the Java API source code.
1.41
Reformat the following program according to the programming style and documentation guidelines. Use the end-of-line brace style.
public class Test
{
// Main method
public static void main(String[] args) {
/** Display output */
System.out.println("Welcome to Java");
}
}
✓
Check
Point
20 Chapter 1
Introduction to Computers, Programs, and Java
1.10 Programming Errors
Key
Point
Programming errors can be categorized into three types: syntax errors, runtime
errors, and logic errors.
1.10.1
Syntax Errors
Errors that are detected by the compiler are called syntax errors or compile errors. Syntax
errors result from errors in code construction, such as mistyping a keyword, omitting some
necessary punctuation, or using an opening brace without a corresponding closing brace.
These errors are usually easy to detect because the compiler tells you where they are and
what caused them. For example, the program in Listing 1.4 has a syntax error, as shown in
Figure 1.10.
syntax errors
compile errors
LISTING 1.4 ShowSyntaxErrors.java
1
2
3
4
5
public class ShowSyntaxErrors {
public static main(String[] args) {
System.out.println("Welcome to Java);
}
}
Four errors are reported, but the program actually has two errors:
■
The keyword void is missing before main in line 2.
■
The string Welcome to Java should be closed with a closing quotation mark in line 3.
Since a single error will often display many lines of compile errors, it is a good practice to
fix errors from the top line and work downward. Fixing errors that occur earlier in the program
may also fix additional errors that occur later.
Compile
FIGURE 1.10
The compiler reports syntax errors.
Tip
If you don’t know how to correct it, compare your program closely, character by character, with similar examples in the text. In the first few weeks of this course, you will
probably spend a lot of time fixing syntax errors. Soon you will be familiar with Java
syntax and can quickly fix syntax errors.
fix syntax errors
1.10.2
runtime errors
Runtime Errors
Runtime errors are errors that cause a program to terminate abnormally. They occur while
a program is running if the environment detects an operation that is impossible to carry out.
Input mistakes typically cause runtime errors. An input error occurs when the program is
1.10 Programming Errors 21
waiting for the user to enter a value, but the user enters a value that the program cannot handle.
For instance, if the program expects to read in a number, but instead the user enters a string,
this causes data-type errors to occur in the program.
Another example of runtime errors is division by zero. This happens when the divisor is
zero for integer divisions. For instance, the program in Listing 1.5 would cause a runtime
error, as shown in Figure 1.11.
LISTING 1.5 ShowRuntimeErrors.java
1
2
3
4
5
public class ShowRuntimeErrors {
public static void main(String[] args) {
System.out.println(1 / 0);
}
}
runtime error
Run
FIGURE 1.11
1.10.3
The runtime error causes the program to terminate abnormally.
Logic Errors
Logic errors occur when a program does not perform the way it was intended to. Errors of
this kind occur for many different reasons. For example, suppose you wrote the program in
Listing 1.6 to convert Celsius 35 degrees to a Fahrenheit degree:
LISTING 1.6 ShowLogicErrors.java
1
2
3
4
5
6
public class ShowLogicErrors {
public static void main(String[] args) {
System.out.println("Celsius 35 is Fahrenheit degree ");
System.out.println((9 / 5) * 35 + 32);
}
}
Celsius 35 is Fahrenheit degree
67
You will get Fahrenheit 67 degrees, which is wrong. It should be 95.0. In Java, the division for integers is the quotient—the fractional part is truncated—so in Java 9 / 5 is 1. To
get the correct result, you need to use 9.0 / 5, which results in 1.8.
In general, syntax errors are easy to find and easy to correct because the compiler gives
indications as to where the errors came from and why they are wrong. Runtime errors are not
difficult to find, either, since the reasons and locations for the errors are displayed on the console
when the program aborts. Finding logic errors, on the other hand, can be very challenging. In the
upcoming chapters, you will learn the techniques of tracing programs and finding logic errors.
1.10.4
Common Errors
Missing a closing brace, missing a semicolon, missing quotation marks for strings, and misspelling names are common errors for new programmers.
logic errors
22 Chapter 1
Introduction to Computers, Programs, and Java
Common Error 1: Missing Braces
The braces are used to denote a block in the program. Each opening brace must be matched
by a closing brace. A common error is missing the closing brace. To avoid this error, type a
closing brace whenever an opening brace is typed, as shown in the following example.
public class Welcome {
}
Type this closing brace right away to match the opening brace
If you use an IDE such as NetBeans and Eclipse, the IDE automatically inserts a closing
brace for each opening brace typed.
Common Error 2: Missing Semicolons
Each statement ends with a statement terminator (;). Often, a new programmer forgets to place
a statement terminator for the last statement in a block, as shown in the following example.
public static void main(String[] args) {
System.out.println("Programming is fun!");
System.out.println("Fundamentals First");
System.out.println("Problem Driven")
}
Missing a semicolon
Common Error 3: Missing Quotation Marks
A string must be placed inside the quotation marks. Often, a new programmer forgets to place
a quotation mark at the end of a string, as shown in the following example.
System.out.println("Problem Driven );
Missing a quotation mark
If you use an IDE such as NetBeans and Eclipse, the IDE automatically inserts a closing
quotation mark for each opening quotation mark typed.
Common Error 4: Misspelling Names
Java is case sensitive. Misspelling names is a common error for new programmers. For example, the word main is misspelled as Main and String is misspelled as string in the following code.
1
2
3
4
5
✓
Check
Point
1.42
1.43
1.44
1.45
1.46
public class Test {
public static void Main(string[] args) {
System.out.println((10.5 + 2 * 3) / (45 – 3.5));
}
}
What are syntax errors (compile errors), runtime errors, and logic errors?
Give examples of syntax errors, runtime errors, and logic errors.
If you forget to put a closing quotation mark on a string, what kind error will be
raised?
If your program needs to read integers, but the user entered strings, an error would
occur when running this program. What kind of error is this?
Suppose you write a program for computing the perimeter of a rectangle and you
mistakenly write your program so that it computes the area of a rectangle. What kind
of error is this?
1.11 Developing Java Programs Using NetBeans 23
1.47
Identify and fix the errors in the following code:
1
2
3
4
5
public class Welcome {
public void Main(String[] args) {
System.out.println('Welcome to Java!);
}
}
1.11 Developing Java Programs Using NetBeans
You can edit, compile, run, and debug Java Programs using NetBeans.
NetBeans and Eclipse are two free popular integrated development environments for developing Java programs. They are easy to learn if you follow simple instructions. We recommend that you use either one for developing Java programs. This section gives the essential
instructions to guide new users to create a project, create a class, compile, and run a class in
NetBeans. The use of Eclipse will be introduced in the next section. For instructions on downloading and installing latest version of NetBeans, see Supplement II.B.
1.11.1
Creating a Java Project
Before you can create Java programs, you need to first create a project. A project is like a
folder to hold Java programs and all supporting files. You need to create a project only once.
Here are the steps to create a Java project:
1. Choose File, New Project to display the New Project dialog box, as shown in Figure 1.12.
2. Select Java in the Categories section and Java Application in the Projects section and
click Next to display the New Java Application dialog box, as shown in Figure 1.13.
3. Type demo in the Project Name field and c:\michael in Project Location field. Uncheck
Use Dedicated Folder for Storing Libraries and uncheck Create Main Class.
4. Click Finish to create the project, as shown in Figure 1.14.
1.11.2
Creating a Java Class
After a project is created, you can create Java programs in the project using the following
steps:
1. Right-click the demo node in the project pane to display a context menu. Choose New,
Java Class to display the New Java Class dialog box, as shown in Figure 1.15.
FIGURE 1.12 The New Project dialog is used to create a new project and specify a project type.
Key
Point
VideoNote
NetBeans brief tutorial
24 Chapter 1
Introduction to Computers, Programs, and Java
FIGURE 1.13 The New Java Application dialog is for specifying a project name and location.
FIGURE 1.14
A New Java project named demo is created.
FIGURE 1.15
The New Java Class dialog box is used to create a new Java class.
2. Type Welcome in the Class Name field and select the Source Packages in the Location
field. Leave the Package field blank. This will create a class in the default package.
3. Click Finish to create the Welcome class. The source code file Welcome.java is placed
under the <default package> node.
4. Modify the code in the Welcome class to match Listing 1.1 in the text, as shown in Figure 1.16.
1.12 Developing Java Programs Using Eclipse 25
Edit pane
Output pane
FIGURE 1.16 You can edit a program and run it in NetBeans.
1.11.3
Compiling and Running a Class
To run Welcome.java, right-click Welcome.java to display a context menu and choose Run File,
or simply press Shift + F6. The output is displayed in the Output pane, as shown in Figure 1.16.
The Run File command automatically compiles the program if the program has been changed.
1.12 Developing Java Programs Using Eclipse
You can edit, compile, run, and debug Java Programs using Eclipse.
The preceding section introduced developing Java programs using NetBeans. You can also
use Eclipse to develop Java programs. This section gives the essential instructions to guide
new users to create a project, create a class, and compile/run a class in Eclipse. For instructions on downloading and installing latest version of Eclipse, see Supplement II.D.
1.12.1
Key
Point
Creating a Java Project
Before creating Java programs in Eclipse, you need to first create a project to hold all files.
VideoNote
Here are the steps to create a Java project in Eclipse:
Eclipse brief tutorial
1. Choose File, New, Java Project to display the New Project wizard, as shown in Figure 1.17.
2. Type demo in the Project name field. As you type, the Location field is automatically set
by default. You may customize the location for your project.
3. Make sure that you selected the options Use project folder as root for sources and class
files so that the .java and .class files are in the same folder for easy access.
4. Click Finish to create the project, as shown in Figure 1.18.
1.12.2
Creating a Java Class
After a project is created, you can create Java programs in the project using the following steps:
1. Choose File, New, Class to display the New Java Class wizard.
2. Type Welcome in the Name field.
26 Chapter 1
Introduction to Computers, Programs, and Java
FIGURE 1.17
The New Java Project dialog is for specifying a project name and properties.
FIGURE 1.18
A New Java project named demo is created.
3. Check the option public static void main(String[] args).
4. Click Finish to generate the template for the source code Welcome.java, as shown in
Figure 1.19.
1.12 Developing Java Programs Using Eclipse 27
FIGURE 1.19
1.12.3
The New Java Class dialog box is used to create a new Java class.
Compiling and Running a Class
To run the program, right-click the class in the project to display a context menu. Choose Run,
Java Application in the context menu to run the class. The output is displayed in the Console
pane, as shown in Figure 1.20.
Edit pane
Output pane
FIGURE 1.20
You can edit a program and run it in Eclipse.
28 Chapter 1
Introduction to Computers, Programs, and Java
KEY TERMS
Application Program Interface (API) 11
assembler 7
assembly language 7
bit 3
block 13
block comment 13
bus 2
byte 3
bytecode 16
bytecode verifier 17
cable modem 6
central processing unit (CPU) 3
class loader 17
comment 13
compiler 8
console 12
dot pitch 6
DSL (digital subscriber line) 6
encoding scheme 3
hardware 2
high-level language 8
integrated development environment
(IDE) 12
interpreter 8
java command 17
Java Development Toolkit (JDK) 12
Java language specification 11
Java Virtual Machine (JVM) 16
17
keyword (or reserved word) 13
library 11
line comment 13
logic error 21
low-level language 7
machine language 7
main method 13
memory 4
modem 00
motherboard 3
network interface card (NIC) 6
operating system (OS) 9
pixel 6
program 2
programming 2
runtime error 20
screen resolution 6
software 2
source code 8
source program 8
statement 8
statement terminator 13
storage devices 4
syntax error 20
javac command
Note
Supplement I.A
The above terms are defined in this chapter. Supplement I.A, Glossary, lists all the key
terms and descriptions in the book, organized by chapters.
CHAPTER SUMMARY
1. A computer is an electronic device that stores and processes data.
2. A computer includes both hardware and software.
3. Hardware is the physical aspect of the computer that can be touched.
4. Computer programs, known as software, are the invisible instructions that control the
hardware and make it perform tasks.
5. Computer programming is the writing of instructions (i.e., code) for computers to perform.
6. The central processing unit (CPU) is a computer’s brain. It retrieves instructions from
memory and executes them.
7. Computers use zeros and ones because digital devices have two stable states, referred to
by convention as zero and one.
Chapter Summary 29
8. A bit is a binary digit 0 or 1.
9. A byte is a sequence of 8 bits.
10. A kilobyte is about 1,000 bytes, a megabyte about 1 million bytes, a gigabyte about 1
billion bytes, and a terabyte about 1,000 gigabytes.
11. Memory stores data and program instructions for the CPU to execute.
12. A memory unit is an ordered sequence of bytes.
13. Memory is volatile, because information is lost when the power is turned off.
14. Programs and data are permanently stored on storage devices and are moved to memory
when the computer actually uses them.
15. The machine language is a set of primitive instructions built into every computer.
16. Assembly language is a low-level programming language in which a mnemonic is used
to represent each machine-language instruction.
17. High-level languages are English-like and easy to learn and program.
18. A program written in a high-level language is called a source program.
19. A compiler is a software program that translates the source program into a machinelanguage program.
20. The operating system (OS) is a program that manages and controls a computer’s activities.
21. Java is platform independent, meaning that you can write a program once and run it on
any computer.
22. Java programs can be embedded in HTML pages and downloaded by Web browsers to
bring live animation and interaction to Web clients.
23. The Java source file name must match the public class name in the program. Java source
code files must end with the .java extension.
24. Every class is compiled into a separate bytecode file that has the same name as the class
and ends with the .class extension.
25. To compile a Java source-code file from the command line, use the javac command.
26. To run a Java class from the command line, use the java command.
27. Every Java program is a set of class definitions. The keyword class introduces a class
definition. The contents of the class are included in a block.
28. A block begins with an opening brace ({) and ends with a closing brace (}).
29. Methods are contained in a class. To run a Java program, the program must have a
main method. The main method is the entry point where the program starts when it is
executed.
30 Chapter 1
Introduction to Computers, Programs, and Java
30. Every statement in Java ends with a semicolon (;), known as the statement terminator.
31. Reserved words, or keywords, have a specific meaning to the compiler and cannot be
used for other purposes in the program.
32. In Java, comments are preceded by two slashes (//) on a line, called a line comment, or
enclosed between /* and */ on one or several lines, called a block comment or paragraph comment. Comments are ignored by the compiler.
33. Java source programs are case sensitive.
34. Programming errors can be categorized into three types: syntax errors, runtime errors,
and logic errors. Errors reported by a compiler are called syntax errors or compile
errors. Runtime errors are errors that cause a program to terminate abnormally. Logic
errors occur when a program does not perform the way it was intended to.
QUIZ
Answer the quiz for this chapter online at www.cs.armstrong.edu/liang/intro10e/quiz.html.
PROGRAMMING EXERCISES
Note
Solutions to even-numbered programming exercises are on the Companion Website.
Solutions to all programming exercises are on the Instructor Resource Website.
Additional programming exercises with solutions are provided to the instructors on the
Instructor Resource Website. The level of difficulty is rated easy (no star), moderate (*),
hard (**), or challenging (***).
level of difficulty
1.1
1.2
*1.3
(Display three messages) Write a program that displays Welcome to Java,
Welcome to Computer Science, and Programming is fun.
(Display five messages) Write a program that displays Welcome to Java five times.
(Display a pattern) Write a program that displays the following pattern:
J
J
J
J
J J
1.4
1.6
A
V
V
V
V V
V
A
A
A A
AAAAA
V
A
A
(Print a table) Write a program that displays the following table:
a
1
2
3
4
1.5
A
A A
AAAAA
a^2
1
4
9
16
a^3
1
8
27
64
(Compute expressions) Write a program that displays the result of
9.5 * 4.5 - 2.5 * 3
.
45.5 - 3.5
(Summation of a series) Write a program that displays the result of
1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9.
Programming Exercises 31
1.7
(Approximatep) p can be computed using the following formula:
p = 4 * ¢1 -
1
1
1
1
1
+ - + + c≤
3
5
7
9
11
Write a program that displays the result of 4 * ¢ 1 and 4 * ¢ 1 -
1
1
1
1
1
+ - + ≤
3
5
7
9
11
1
1
1
1
1
1
+ - + +
≤. Use 1.0 instead of 1 in your
3
5
7
9
11
13
program.
1.8
(Area and perimeter of a circle) Write a program that displays the area and perimeter of a circle that has a radius of 5.5 using the following formula:
perimeter = 2 * radius * p
area = radius * radius * p
1.9
(Area and perimeter of a rectangle) Write a program that displays the area and
perimeter of a rectangle with the width of 4.5 and height of 7.9 using the following
formula:
area = width * height
1.10
(Average speed in miles) Assume a runner runs 14 kilometers in 45 minutes and 30
seconds. Write a program that displays the average speed in miles per hour. (Note
that 1 mile is 1.6 kilometers.)
(Population projection) The U.S. Census Bureau projects population based on the
following assumptions:
*1.11
■
■
■
1.12
*1.13
One birth every 7 seconds
One death every 13 seconds
One new immigrant every 45 seconds
Write a program to display the population for each of the next five years. Assume the
current population is 312,032,486 and one year has 365 days. Hint: In Java, if two
integers perform division, the result is an integer. The fractional part is truncated. For
example, 5 / 4 is 1 (not 1.25) and 10 / 4 is 2 (not 2.5). To get an accurate result with
the fractional part, one of the values involved in the division must be a number with a
decimal point. For example, 5.0 / 4 is 1.25 and 10 / 4.0 is 2.5.
(Average speed in kilometers) Assume a runner runs 24 miles in 1 hour, 40 minutes,
and 35 seconds. Write a program that displays the average speed in kilometers per
hour. (Note that 1 mile is 1.6 kilometers.)
(Algebra: solve 2 * 2 linear equations) You can use Cramer’s rule to solve the following 2 * 2 system of linear equation:
ed - bf
af - ec
ax + by = e
x =
y =
cx + dy = f
ad - bc
ad - bc
Write a program that solves the following equation and displays the value for x and y :
3.4x + 50.2y = 44.5
2.1x + .55y = 5.9
This page intentionally left blank
CHAPTER
ELEMENTARY
PROGRAMMING
Objectives
■
■
■
■
■
■
■
■
■
■
■
■
■
■
■
■
■
■
■
■
To write Java programs to perform simple computations (§2.2).
To obtain input from the console using the Scanner class (§2.3).
To use identifiers to name variables, constants, methods, and classes (§2.4).
To use variables to store data (§§2.5–2.6).
To program with assignment statements and assignment expressions
(§2.6).
To use constants to store permanent data (§2.7).
To name classes, methods, variables, and constants by following their
naming conventions (§2.8).
To explore Java numeric primitive data types: byte, short, int,
long, float, and double (§2.9.1).
To read a byte, short, int, long, float, or double value from the
keyboard (§2.9.2).
To perform operations using operators +, -, *, /, and % (§2.9.3).
To perform exponent operations using Math.pow(a, b) (§2.9.4).
To write integer literals, floating-point literals, and literals in scientific
notation (§2.10).
To write and evaluate numeric expressions (§2.11).
To obtain the current system time using
System.currentTimeMillis() (§2.12).
To use augmented assignment operators (§2.13).
To distinguish between postincrement and preincrement and between
postdecrement and predecrement (§2.14).
To cast the value of one type to another type (§2.15).
To describe the software development process and apply it to develop
the loan payment program (§2.16).
To write a program that converts a large amount of money into smaller
units (§2.17).
To avoid common errors and pitfalls in elementary programming (§2.18).
2
34 Chapter 2
Elementary Programming
2.1 Introduction
Key
Point
The focus of this chapter is on learning elementary programming techniques to solve
problems.
In Chapter 1 you learned how to create, compile, and run very basic Java programs. Now you
will learn how to solve problems by writing programs. Through these problems, you will learn
elementary programming using primitive data types, variables, constants, operators, expressions, and input and output.
Suppose, for example, that you need to take out a student loan. Given the loan amount, loan
term, and annual interest rate, can you write a program to compute the monthly payment and
total payment? This chapter shows you how to write programs like this. Along the way, you
learn the basic steps that go into analyzing a problem, designing a solution, and implementing
the solution by creating a program.
2.2 Writing a Simple Program
Key
Point
problem
algorithm
pseudocode
Writing a program involves designing a strategy for solving the problem and then
using a programming language to implement that strategy.
Let’s first consider the simple problem of computing the area of a circle. How do we write a
program for solving this problem?
Writing a program involves designing algorithms and translating algorithms into programming instructions, or code. An algorithm describes how a problem is solved by listing
the actions that need to be taken and the order of their execution. Algorithms can help the
programmer plan a program before writing it in a programming language. Algorithms can be
described in natural languages or in pseudocode (natural language mixed with some programming code). The algorithm for calculating the area of a circle can be described as follows:
1. Read in the circle’s radius.
2. Compute the area using the following formula:
area = radius * radius * p
3. Display the result.
Tip
It’s always good practice to outline your program (or its underlying problem) in the form
of an algorithm before you begin coding.
When you code—that is, when you write a program—you translate an algorithm into a program. You already know that every Java program begins with a class definition in which the
keyword class is followed by the class name. Assume that you have chosen ComputeArea
as the class name. The outline of the program would look like this:
public class ComputeArea {
// Details to be given later
}
As you know, every Java program must have a main method where program execution
begins. The program is then expanded as follows:
public class ComputeArea {
public static void main(String[] args) {
// Step 1: Read in radius
// Step 2: Compute area
2.2 Writing a Simple Program 35
// Step 3: Display the area
}
}
The program needs to read the radius entered by the user from the keyboard. This raises
two important issues:
■
Reading the radius.
■
Storing the radius in the program.
Let’s address the second issue first. In order to store the radius, the program needs to declare
a symbol called a variable. A variable represents a value stored in the computer’s memory.
Rather than using x and y as variable names, choose descriptive names: in this case, radius
for radius, and area for area. To let the compiler know what radius and area are, specify their
data types. That is the kind of data stored in a variable, whether integer, real number, or something else. This is known as declaring variables. Java provides simple data types for representing integers, real numbers, characters, and Boolean types. These types are known as primitive
data types or fundamental types.
Real numbers (i.e., numbers with a decimal point) are represented using a method known
as floating-point in computers. So, the real numbers are also called floating-point numbers. In
Java, you can use the keyword double to declare a floating-point variable. Declare radius
and area as double. The program can be expanded as follows:
public class ComputeArea {
public static void main(String[] args) {
double radius;
double area;
// Step 1: Read in radius
// Step 2: Compute area
// Step 3: Display the area
}
}
The program declares radius and area as variables. The reserved word double indicates
that radius and area are floating-point values stored in the computer.
The first step is to prompt the user to designate the circle’s radius. You will soon learn
how to prompt the user for information. For now, to learn how variables work, you can assign
a fixed value to radius in the program as you write the code; later, you’ll modify the program
to prompt the user for this value.
The second step is to compute area by assigning the result of the expression radius *
radius * 3.14159 to area.
In the final step, the program will display the value of area on the console by using the
System.out.println method.
Listing 2.1 shows the complete program, and a sample run of the program is shown in
Figure 2.1.
LISTING 2.1 ComputeArea.java
1
2
3
4
5
6
public class ComputeArea {
public static void main(String[] args) {
double radius; // Declare radius
double area; // Declare area
// Assign a radius
variable
descriptive names
data type
declare variables
primitive data types
floating-point number
36 Chapter 2
Elementary Programming
7
8
9
10
11
12
13
14
15
16
radius = 20; // radius is now 20
// Compute area
area = radius * radius * 3.14159;
// Display results
System.out.println("The area for the circle of radius " +
radius + " is " + area);
}
}
Compile
Run
FIGURE 2.1
declare variable
assign value
tracing program
The program displays the area of a circle.
Variables such as radius and area correspond to memory locations. Every variable has
a name, a type, a size, and a value. Line 3 declares that radius can store a double value.
The value is not defined until you assign a value. Line 7 assigns 20 into variable radius.
Similarly, line 4 declares variable area, and line 10 assigns a value into area. The following
table shows the value in the memory for area and radius as the program is executed. Each
row in the table shows the values of variables after the statement in the corresponding line in
the program is executed. This method of reviewing how a program works is called tracing a
program. Tracing programs are helpful for understanding how programs work, and they are
useful tools for finding errors in programs.
line#
radius
no value
4
7
20
10
concatenate strings
concatenate strings with
numbers
area
no value
3
1256.636
The plus sign (+) has two meanings: one for addition and the other for concatenating (combining) strings. The plus sign (+) in lines 13–14 is called a string concatenation operator. It
combines two strings into one. If a string is combined with a number, the number is converted
into a string and concatenated with the other string. Therefore, the plus signs (+) in lines
13–14 concatenate strings into a longer string, which is then displayed in the output. Strings
and string concatenation will be discussed further in Chapter 4.
Caution
A string cannot cross lines in the source code. Thus, the following statement would
result in a compile error:
System.out.println("Introduction to Java Programming,
by Y. Daniel Liang");
break a long string
To fix the error, break the string into separate substrings, and use the concatenation
operator (+) to combine them:
System.out.println("Introduction to Java Programming, " +
"by Y. Daniel Liang");
2.3 Reading Input from the Console 37
2.1
Identify and fix the errors in the following code:
1
2
3
4
5
6
7
8
9
10
public class Test {
public void main(string[] args) {
double i = 50.0;
double k = i + 50.0;
double j = k + 1;
✓
Check
Point
System.out.println("j is " + j + " and
k is " + k);
}
}
2.3 Reading Input from the Console
Reading input from the console enables the program to accept input from the user.
In Listing 2.1, the radius is fixed in the source code. To use a different radius, you have to
modify the source code and recompile it. Obviously, this is not convenient, so instead you can
use the Scanner class for console input.
Java uses System.out to refer to the standard output device and System.in to the
standard input device. By default, the output device is the display monitor and the input
device is the keyboard. To perform console output, you simply use the println method to
display a primitive value or a string to the console. Console input is not directly supported
in Java, but you can use the Scanner class to create an object to read input from System.in,
as follows:
Key
Point
VideoNote
Obtain input
Scanner input = new Scanner(System.in);
The syntax new Scanner(System.in) creates an object of the Scanner type. The syntax
Scanner input declares that input is a variable whose type is Scanner. The whole line
Scanner input = new Scanner(System.in) creates a Scanner object and assigns its
reference to the variable input. An object may invoke its methods. To invoke a method on
an object is to ask the object to perform a task. You can invoke the nextDouble() method to
read a double value as follows:
double radius = input.nextDouble();
This statement reads a number from the keyboard and assigns the number to radius.
Listing 2.2 rewrites Listing 2.1 to prompt the user to enter a radius.
LISTING 2.2 ComputeAreaWithConsoleInput.java
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
import java.util.Scanner; // Scanner is in the java.util package
import class
public class ComputeAreaWithConsoleInput {
public static void main(String[] args) {
// Create a Scanner object
Scanner input = new Scanner(System.in);
create a Scanner
// Prompt the user to enter a radius
System.out.print("Enter a number for radius: ");
double radius = input.nextDouble();
// Compute area
double area = radius * radius * 3.14159;
// Display results
read a double
38 Chapter 2
Elementary Programming
16
17
18
19
System.out.println("The area for the circle of radius " +
radius + " is " + area);
}
}
Enter a number for radius: 2.5
The area for the circle of radius 2.5 is 19.6349375
Enter a number for radius: 23
The area for the circle of radius 23.0 is 1661.90111
prompt
Line 9 displays a string "Enter a number for radius: " to the console. This is
known as a prompt, because it directs the user to enter an input. Your program should always
tell the user what to enter when expecting input from the keyboard.
The print method in line 9
System.out.print("Enter a number for radius: ");
print vs. println
is identical to the println method except that println moves to the beginning of the next
line after displaying the string, but print does not advance to the next line when completed.
Line 6 creates a Scanner object. The statement in line 10 reads input from the keyboard.
double radius = input.nextDouble();
specific import
After the user enters a number and presses the Enter key, the program reads the number
and assigns it to radius.
More details on objects will be introduced in Chapter 9. For the time being, simply accept
that this is how to obtain input from the console.
The Scanner class is in the java.util package. It is imported in line 1. There are two
types of import statements: specific import and wildcard import. The specific import specifies a single class in the import statement. For example, the following statement imports
Scanner from the package java.util.
import java.util.Scanner;
wildcard import
The wildcard import imports all the classes in a package by using the asterisk as the
wildcard. For example, the following statement imports all the classes from the package
java.util.
import java.uitl.*;
no performance difference
The information for the classes in an imported package is not read in at compile time or
runtime unless the class is used in the program. The import statement simply tells the compiler
where to locate the classes. There is no performance difference between a specific import and
a wildcard import declaration.
Listing 2.3 gives an example of reading multiple input from the keyboard. The program
reads three numbers and displays their average.
LISTING 2.3 ComputeAverage.java
import class
create a Scanner
1
2
3
4
5
6
7
import java.util.Scanner; // Scanner is in the java.util package
public class ComputeAverage {
public static void main(String[] args) {
// Create a Scanner object
Scanner input = new Scanner(System.in);
2.4 Identifiers 39
8
9
10
11
12
13
14
15
16
17
18
19
20
21
// Prompt the user to enter three numbers
System.out.print("Enter three numbers: ");
double number1 = input.nextDouble();
double number2 = input.nextDouble();
double number3 = input.nextDouble();
read a double
// Compute average
double average = (number1 + number2 + number3) / 3;
// Display results
System.out.println("The average of " + number1 + " " + number2
+ " " + number3 + " is " + average);
}
}
Enter three numbers: 1 2 3
The average of 1.0 2.0 3.0 is 2.0
Enter three numbers: 10.5
11
11.5
The average of 10.5 11.0 11.5 is 11.0
The code for importing the Scanner class (line 1) and creating a Scanner object (line
6) are the same as in the preceding example as well as in all new programs you will write for
reading input from the keyboard.
Line 9 prompts the user to enter three numbers. The numbers are read in lines 10–12. You
may enter three numbers separated by spaces, then press the Enter key, or enter each number
followed by a press of the Enter key, as shown in the sample runs of this program.
If you entered an input other than a numeric value, a runtime error would occur. In Chapter 12,
you will learn how to handle the exception so that the program can continue to run.
enter input in one line
enter input in multiple lines
runtime error
Note
Most of the programs in the early chapters of this book perform three steps—input,
process, and output—called IPO. Input is receiving input from the user; process is producing results using the input; and output is displaying the results.
2.2
How do you write a statement to let the user enter a double value from the keyboard?
What happens if you entered 5a when executing the following code?
double radius = input.nextDouble();
2.3
IPO
✓
Check
Point
Are there any performance differences between the following two import statements?
import java.util.Scanner;
import java.util.*;
2.4 Identifiers
Identifiers are the names that identify the elements such as classes, methods, and
variables in a program.
As you see in Listing 2.3, ComputeAverage, main, input, number1, number2, number3,
and so on are the names of things that appear in the program. In programming terminology,
such names are called identifiers. All identifiers must obey the following rules:
■
An identifier is a sequence of characters that consists of letters, digits, underscores
(_), and dollar signs ($).
Key
Point
identifiers
identifier naming rules
40 Chapter 2
Elementary Programming
■
An identifier must start with a letter, an underscore (_), or a dollar sign ($). It cannot
start with a digit.
■
An identifier cannot be a reserved word. (See Appendix A for a list of reserved words.)
■
An identifier cannot be true, false, or null.
■
An identifier can be of any length.
For example, $2, ComputeArea, area, radius, and print are legal identifiers, whereas
2A and d+4 are not because they do not follow the rules. The Java compiler detects illegal
identifiers and reports syntax errors.
Note
Since Java is case sensitive, area, Area, and AREA are all different identifiers.
case sensitive
Tip
Identifiers are for naming variables, methods, classes, and other items in a program.
Descriptive identifiers make programs easy to read. Avoid using abbreviations for identifiers. Using complete words is more descriptive. For example, numberOfStudents
is better than numStuds, numOfStuds, or numOfStudents. We use descriptive
names for complete programs in the text. However, we will occasionally use variable
names such as i, j, k, x, and y in the code snippets for brevity. These names also
provide a generic tone to the code snippets.
descriptive names
Tip
Do not name identifiers with the $ character. By convention, the $ character should be
used only in mechanically generated source code.
the $ character
✓
Check
Point
2.4
Which of the following identifiers are valid? Which are Java keywords?
miles, Test, a++, ––a, 4#R, $4, #44, apps
class, public, int, x, y, radius
2.5 Variables
Key
Point
why called variables?
Variables are used to represent values that may be changed in the program.
As you see from the programs in the preceding sections, variables are used to store values
to be used later in a program. They are called variables because their values can be changed.
In the program in Listing 2.2, radius and area are variables of the double type. You can
assign any numerical value to radius and area, and the values of radius and area can
be reassigned. For example, in the following code, radius is initially 1.0 (line 2) and then
changed to 2.0 (line 7), and area is set to 3.14159 (line 3) and then reset to 12.56636
(line 8).
1
2
3
4
5
6
7
8
9
// Compute the first area
radius = 1.0;
radius: 1.0
area = radius * radius * 3.14159;
area: 3.14159
System.out.println("The area is " + area + " for radius " + radius);
// Compute the second area
radius = 2.0;
radius: 2.0
area = radius * radius * 3.14159;
area: 12.56636
System.out.println("The area is " + area + " for radius " + radius);
Variables are for representing data of a certain type. To use a variable, you declare it by
telling the compiler its name as well as what type of data it can store. The variable declaration
2.6 Assignment Statements and Assignment Expressions 41
tells the compiler to allocate appropriate memory space for the variable based on its data type.
The syntax for declaring a variable is
datatype variableName;
Here are some examples of variable declarations:
declare variable
int count;
// Declare count to be an integer variable
double radius;
// Declare radius to be a double variable
double interestRate; // Declare interestRate to be a double variable
These examples use the data types int and double. Later you will be introduced to additional data types, such as byte, short, long, float, char, and boolean.
If variables are of the same type, they can be declared together, as follows:
datatype variable1, variable2, ..., variablen;
The variables are separated by commas. For example,
int i, j, k; // Declare i, j, and k as int variables
Variables often have initial values. You can declare a variable and initialize it in one step.
Consider, for instance, the following code:
initialize variables
int count = 1;
This is equivalent to the next two statements:
int count;
count = 1;
You can also use a shorthand form to declare and initialize variables of the same type
together. For example,
int i = 1, j = 2;
Tip
A variable must be declared before it can be assigned a value. A variable declared in a
method must be assigned a value before it can be used.
Whenever possible, declare a variable and assign its initial value in one step. This will
make the program easy to read and avoid programming errors.
Every variable has a scope. The scope of a variable is the part of the program where the
variable can be referenced. The rules that define the scope of a variable will be introduced
gradually later in the book. For now, all you need to know is that a variable must be declared
and initialized before it can be used.
2.5
Identify and fix the errors in the following code:
1
2
3
4
5
6
public class Test {
public static void main(String[] args) {
int i = k + 2;
System.out.println(i);
}
}
✓
Check
Point
2.6 Assignment Statements and Assignment
Expressions
An assignment statement designates a value for a variable. An assignment statement
can be used as an expression in Java.
Key
Point
42 Chapter 2
assignment statement
assignment operator
Elementary Programming
After a variable is declared, you can assign a value to it by using an assignment statement. In
Java, the equal sign (=) is used as the assignment operator. The syntax for assignment statements is as follows:
variable = expression;
expression
An expression represents a computation involving values, variables, and operators that,
taking them together, evaluates to a value. For example, consider the following code:
int y = 1;
//
double radius = 1.0;
//
int x = 5 * (3 / 2);
//
x = y + 1;
//
double area = radius * radius *
Assign 1 to variable y
Assign 1.0 to variable radius
Assign the value of the expression to x
Assign the addition of y and 1 to x
3.14159; // Compute area
You can use a variable in an expression. A variable can also be used in both sides of the =
operator. For example,
x = x + 1;
In this assignment statement, the result of x + 1 is assigned to x. If x is 1 before the statement is executed, then it becomes 2 after the statement is executed.
To assign a value to a variable, you must place the variable name to the left of the assignment operator. Thus, the following statement is wrong:
1 = x;
// Wrong
Note
In mathematics, x = 2 * x + 1 denotes an equation. However, in Java, x = 2 * x
+ 1 is an assignment statement that evaluates the expression 2 * x + 1 and assigns
the result to x.
assignment expression
In Java, an assignment statement is essentially an expression that evaluates to the value
to be assigned to the variable on the left side of the assignment operator. For this reason, an
assignment statement is also known as an assignment expression. For example, the following
statement is correct:
System.out.println(x = 1);
which is equivalent to
x = 1;
System.out.println(x);
If a value is assigned to multiple variables, you can use this syntax:
i = j = k = 1;
which is equivalent to
k = 1;
j = k;
i = j;
Note
In an assignment statement, the data type of the variable on the left must be compatible with the data type of the value on the right. For example, int x = 1.0 would be
2.7 Named Constants 43
illegal, because the data type of x is int. You cannot assign a double value (1.0) to
an int variable without using type casting. Type casting is introduced in Section 2.15.
2.6
Identify and fix the errors in the following code:
1
2
3
4
5
6
public class Test {
public static void main(String[] args) {
int i = j = k = 2;
System.out.println(i + " " + j + " " + k);
}
}
✓
Check
Point
2.7 Named Constants
A named constant is an identifier that represents a permanent value.
The value of a variable may change during the execution of a program, but a named constant, or simply constant, represents permanent data that never changes. In our ComputeArea
program, p is a constant. If you use it frequently, you don’t want to keep typing 3.14159;
instead, you can declare a constant for p. Here is the syntax for declaring a constant:
Key
Point
constant
final datatype CONSTANTNAME = value;
A constant must be declared and initialized in the same statement. The word final is
a Java keyword for declaring a constant. For example, you can declare p as a constant and
rewrite Listing 2.1 as in Listing 2.4.
final keyword
LISTING 2.4 ComputeAreaWithConstant.java
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
import java.util.Scanner; // Scanner is in the java.util package
public class ComputeAreaWithConstant {
public static void main(String[] args) {
final double PI = 3.14159; // Declare a constant
// Create a Scanner object
Scanner input = new Scanner(System.in);
// Prompt the user to enter a radius
System.out.print("Enter a number for radius: ");
double radius = input.nextDouble();
// Compute area
double area = radius * radius * PI;
// Display result
System.out.println("The area for the circle of radius " +
radius + " is " + area);
}
}
There are three benefits of using constants: (1) you don’t have to repeatedly type the same
value if it is used multiple times; (2) if you have to change the constant value (e.g., from 3.14
to 3.14159 for PI), you need to change it only in a single location in the source code; and
(3) a descriptive name for a constant makes the program easy to read.
benefits of constants
44 Chapter 2
Elementary Programming
2.8 Naming Conventions
Key
Point
Sticking with the Java naming conventions makes your programs easy to read and
avoids errors.
Make sure that you choose descriptive names with straightforward meanings for the variables,
constants, classes, and methods in your program. As mentioned earlier, names are case sensitive. Listed below are the conventions for naming variables, methods, and classes.
name variables and methods
■
Use lowercase for variables and methods. If a name consists of several words, concatenate them into one, making the first word lowercase and capitalizing the first
letter of each subsequent word—for example, the variables radius and area and
the method print.
name classes
■
Capitalize the first letter of each word in a class name—for example, the class names
ComputeArea and System.
name constants
■
Capitalize every letter in a constant, and use underscores between words—for example, the constants PI and MAX_VALUE.
It is important to follow the naming conventions to make your programs easy to read.
Caution
Do not choose class names that are already used in the Java library. For example, since
the System class is defined in Java, you should not name your class System.
name classes
✓
Check
Point
2.7
2.8
2.9
What are the benefits of using constants? Declare an int constant SIZE with value 20.
What are the naming conventions for class names, method names, constants, and
variables? Which of the following items can be a constant, a method, a variable, or a
class according to the Java naming conventions?
MAX_VALUE, Test, read, readDouble
Translate the following algorithm into Java code:
Step 1: Declare a double variable named miles with initial value 100.
Step 2: Declare a double constant named KILOMETERS_PER_MILE with value
1.609.
Step 3: Declare a double variable named kilometers, multiply miles and
KILOMETERS_PER_MILE, and assign the result to kilometers.
Step 4: Display kilometers to the console.
What is kilometers after Step 4?
2.9 Numeric Data Types and Operations
Key
Point
Java has six numeric types for integers and floating-point numbers with operators +,
-, *, /, and %.
2.9.1
Numeric Types
Every data type has a range of values. The compiler allocates memory space for each variable or constant according to its data type. Java provides eight primitive data types for
numeric values, characters, and Boolean values. This section introduces numeric data types
and operators.
Table 2.1 lists the six numeric data types, their ranges, and their storage sizes.
2.9 Numeric Data Types and Operations 45
TABLE 2.1 Numeric Data Types
Name
Range
byte
-2 to 2 - 1 ( -128 to 127)
8-bit signed
byte type
short
-215 to 215 - 1 ( - 32768 to 32767)
16-bit signed
short type
int
-231 to 231 - 1 ( - 2147483648 to 2147483647)
32-bit signed
int type
long
-263 to 263 - 1
64-bit signed
long type
32-bit IEEE 754
float type
64-bit IEEE 754
double type
7
Storage Size
7
(i.e., - 9223372036854775808 to 9223372036854775807)
Negative range: - 3.4028235E + 38 to - 1.4E - 45
float
Positive range: 1.4E - 45 to 3.4028235E + 38
Negative range: - 1.7976931348623157E + 308 to
-4.9E - 324
double
Positive range: 4.9E - 324 to 1.7976931348623157E + 308
Note
IEEE 754 is a standard approved by the Institute of Electrical and Electronics Engineers
for representing floating-point numbers on computers. The standard has been widely
adopted. Java uses the 32-bit IEEE 754 for the float type and the 64-bit IEEE 754
for the double type. The IEEE 754 standard also defines special floating-point values,
which are listed in Appendix E.
Java uses four types for integers: byte, short, int, and long. Choose the type that is
most appropriate for your variable. For example, if you know an integer stored in a variable
is within a range of a byte, declare the variable as a byte. For simplicity and consistency, we
will use int for integers most of the time in this book.
Java uses two types for floating-point numbers: float and double. The double type
is twice as big as float, so the double is known as double precision and float as single
precision. Normally, you should use the double type, because it is more accurate than the
float type.
2.9.2
Reading Numbers from the Keyboard
You know how to use the nextDouble() method in the Scanner class to read a double
value from the keyboard. You can also use the methods listed in Table 2.2 to read a number
of the byte, short, int, long, and float type.
TABLE 2.2 Methods for Scanner Objects
Method
Description
nextByte()
reads an integer of the byte type.
nextShort()
reads an integer of the short type.
nextInt()
reads an integer of the int type.
nextLong()
reads an integer of the long type.
nextFloat()
reads a number of the float type.
nextDouble()
reads a number of the double type.
integer types
floating-point types
46 Chapter 2
Elementary Programming
Here are examples for reading values of various types from the keyboard:
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
Scanner input = new Scanner(System.in);
System.out.print("Enter a byte value: ");
byte byteValue = input.nextByte();
System.out.print("Enter a short value: ");
short shortValue = input.nextShort();
System.out.print("Enter an int value: ");
int intValue = input.nextInt();
System.out.print("Enter a long value: ");
long longValue = input.nextLong();
System.out.print("Enter a float value: ");
float floatValue = input.nextFloat();
If you enter a value with an incorrect range or format, a runtime error would occur. For
example, you enter a value 128 for line 3, an error would occur because 128 is out of range
for a byte type integer.
2.9.3
operators +, -, *, /, %
operands
Numeric Operators
The operators for numeric data types include the standard arithmetic operators: addition (+),
subtraction (–), multiplication (*), division (/), and remainder (%), as shown in Table 2.3. The
operands are the values operated by an operator.
TABLE 2.3 Numeric Operators
integer division
Name
Meaning
Example
Result
+
Addition
34 + 1
35
-
Subtraction
34.0 – 0.1
33.9
*
Multiplication
300 * 30
9000
/
Division
1.0 / 2.0
0.5
%
Remainder
20 % 3
2
When both operands of a division are integers, the result of the division is the quotient
and the fractional part is truncated. For example, 5 / 2 yields 2, not 2.5, and –5 / 2 yields
-2, not –2.5. To perform a float-point division, one of the operands must be a floating-point
number. For example, 5.0 / 2 yields 2.5.
The % operator, known as remainder or modulo operator, yields the remainder after division. The operand on the left is the dividend and the operand on the right is the divisor. Therefore, 7 % 3 yields 1, 3 % 7 yields 3, 12 % 4 yields 0, 26 % 8 yields 2, and 20 % 13 yields 7.
2
3
7
6
1
0
7
3
0
3
3
4
12
12
0
3
8
26
24
2
Divisor
13
1
Quotient
20
Dividend
13
7
Remainder
The % operator is often used for positive integers, but it can also be used with negative integers and floating-point values. The remainder is negative only if the dividend is negative. For
example, -7 % 3 yields -1, -12 % 4 yields 0, -26 % -8 yields -2, and 20 % -13 yields 7.
2.9 Numeric Data Types and Operations 47
Remainder is very useful in programming. For example, an even number % 2 is always 0
and an odd number % 2 is always 1. Thus, you can use this property to determine whether a
number is even or odd. If today is Saturday, it will be Saturday again in 7 days. Suppose you
and your friends are going to meet in 10 days. What day is in 10 days? You can find that the
day is Tuesday using the following expression:
Day 6 in a week is Saturday
A week has 7 days
(6 + 10) % 7 is 2
After 10 days
Day 2 in a week is Tuesday
Note: Day 0 in a week is Sunday
The program in Listing 2.5 obtains minutes and remaining seconds from an amount of time
in seconds. For example, 500 seconds contains 8 minutes and 20 seconds.
LISTING 2.5 DisplayTime.java
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
import java.util.Scanner;
import Scanner
public class DisplayTime {
public static void main(String[] args) {
Scanner input = new Scanner(System.in);
// Prompt the user for input
System.out.print("Enter an integer for seconds: ");
int seconds = input.nextInt();
create a Scanner
int minutes = seconds / 60; // Find minutes in seconds
int remainingSeconds = seconds % 60; // Seconds remaining
System.out.println(seconds + " seconds is " + minutes +
" minutes and " + remainingSeconds + " seconds");
read an integer
divide
remainder
}
}
Enter an integer for seconds: 500
500 seconds is 8 minutes and 20 seconds
line#
8
10
11
seconds
minutes
remainingSeconds
500
8
20
The nextInt() method (line 8) reads an integer for seconds. Line 10 obtains the minutes using seconds / 60. Line 11 (seconds % 60) obtains the remaining seconds after
taking away the minutes.
The + and - operators can be both unary and binary. A unary operator has only one
operand; a binary operator has two. For example, the - operator in -5 is a unary operator
to negate number 5, whereas the - operator in 4 - 5 is a binary operator for subtracting 5
from 4.
unary operator
binary operator
48 Chapter 2
Elementary Programming
2.9.4
Math.pow(a, b) method
Exponent Operations
The Math.pow(a, b) method can be used to compute ab. The pow method is defined in
the Math class in the Java API. You invoke the method using the syntax Math.pow(a, b)
(e.g., Math.pow(2, 3)), which returns the result of ab (23). Here, a and b are parameters
for the pow method and the numbers 2 and 3 are actual values used to invoke the method. For
example,
System.out.println(Math.pow(2, 3)); // Displays 8.0
System.out.println(Math.pow(4, 0.5)); // Displays 2.0
System.out.println(Math.pow(2.5, 2)); // Displays 6.25
System.out.println(Math.pow(2.5, -2)); // Displays 0.16
Chapter 5 introduces more details on methods. For now, all you need to know is how to
invoke the pow method to perform the exponent operation.
✓
Check
Point
2.10
Find the largest and smallest byte, short, int, long, float, and double. Which
of these data types requires the least amount of memory?
2.11
Show the result of the following remainders.
56
78
-34
-34
5
1
% 6
% -4
% 5
% -5
% 1
% 5
2.12
If today is Tuesday, what will be the day in 100 days?
2.13
What is the result of 25 / 4? How would you rewrite the expression if you wished
the result to be a floating-point number?
2.14
Show the result of the following code:
System.out.println(2
System.out.println(2
System.out.println(2
System.out.println(2
2.15
*
*
*
*
(5 / 2 + 5 / 2));
5 / 2 + 2 * 5 / 2);
(5 / 2));
5 / 2);
Are the following statements correct? If so, show the output.
System.out.println("25 / 4 is " + 25 /
System.out.println("25 / 4.0 is " + 25
System.out.println("3 * 2 / 4 is " + 3
System.out.println("3.0 * 2 / 4 is " +
4);
/ 4.0);
* 2 / 4);
3.0 * 2 / 4);
2.16
Write a statement to display the result of 23.5.
2.17
Suppose m and r are integers. Write a Java expression for mr2 to obtain a floatingpoint result.
2.10 Numeric Literals
Key
Point
literal
A literal is a constant value that appears directly in a program.
For example, 34 and 0.305 are literals in the following statements:
int numberOfYears = 34;
double weight = 0.305;
2.10 Numeric Literals 49
2.10.1
Integer Literals
An integer literal can be assigned to an integer variable as long as it can fit into the variable. A
compile error will occur if the literal is too large for the variable to hold. The statement byte
b = 128, for example, will cause a compile error, because 128 cannot be stored in a variable
of the byte type. (Note that the range for a byte value is from –128 to 127.)
An integer literal is assumed to be of the int type, whose value is between
-231 (-2147483648) and 231 - 1 (2147483647). To denote an integer literal of the long
type, append the letter L or l to it. For example, to write integer 2147483648 in a Java program, you have to write it as 2147483648L or 2147483648l, because 2147483648 exceeds
the range for the int value. L is preferred because l (lowercase L) can easily be confused
with 1 (the digit one).
Note
By default, an integer literal is a decimal integer number. To denote a binary integer
literal, use a leading 0b or 0B (zero B), to denote an octal integer literal, use a leading
0 (zero), and to denote a hexadecimal integer literal, use a leading 0x or 0X (zero X).
For example,
binary, octal, and hex literals
System.out.println(0B1111); // Displays 15
System.out.println(07777); // Displays 4095
System.out.println(0XFFFF); // Displays 65535
Hexadecimal numbers, binary numbers, and octal numbers are introduced in Appendix F.
2.10.2
Floating-Point Literals
Floating-point literals are written with a decimal point. By default, a floating-point literal is
treated as a double type value. For example, 5.0 is considered a double value, not a float
value. You can make a number a float by appending the letter f or F, and you can make
a number a double by appending the letter d or D. For example, you can use 100.2f or
100.2F for a float number, and 100.2d or 100.2D for a double number.
suffix f or F
suffix d or D
Note
The double type values are more accurate than the float type values. For example,
System.out.println("1.0 / 3.0 is " + 1.0 / 3.0);
x
displays 1.0 / 3.0 is 0.3333333333333333
16 digits
System.out.println("1.0F / 3.0F is " + 1.0F / 3.0F);
s
displays 1.0F / 3.0F is 0.33333334
8 digits
A float value has 7 to 8 number of significant digits and a double value has 15 to 17 number
of significant digits.
2.10.3
Scientific Notation
Floating-point literals can be written in scientific notation in the form of a * 10b. For example,
the scientific notation for 123.456 is 1.23456 * 102 and for 0.0123456 is 1.23456 * 10-2.
A special syntax is used to write scientific notation numbers. For example, 1.23456 * 102 is
written as 1.23456E2 or 1.23456E+2 and 1.23456 * 10-2 as 1.23456E-2. E (or e) represents an exponent and can be in either lowercase or uppercase.
double vs. float
50 Chapter 2
Elementary Programming
Note
The float and double types are used to represent numbers with a decimal point.
Why are they called floating-point numbers? These numbers are stored in scientific notation internally. When a number such as 50.534 is converted into scientific notation,
such as 5.0534E+1, its decimal point is moved (i.e., floated) to a new position.
why called floating-point?
Note
To improve readability, Java allows you to use underscores between two digits in a
number literal. For example, the following literals are correct.
long ssn = 232_45_4519;
long creditCardNumber = 2324_4545_4519_3415L;
However, 45_ or _45 is incorrect. The underscore must be placed between two digits.
underscores in numbers
✓
Check
Point
2.18
How many accurate digits are stored in a float or double type variable?
2.19
Which of the following are correct literals for floating-point numbers?
12.3, 12.3e+2, 23.4e-2, –334.4, 20.5, 39F, 40D
2.20
Which of the following are the same as 52.534?
5.2534e+1, 0.52534e+2, 525.34e-1, 5.2534e+0
2.21
Which of the following are correct literals?
5_2534e+1, _2534, 5_2, 5_
2.11 Evaluating Expressions and Operator Precedence
Key
Point
Java expressions are evaluated in the same way as arithmetic expressions.
Writing a numeric expression in Java involves a straightforward translation of an arithmetic
expression using Java operators. For example, the arithmetic expression
10(y - 5)(a + b + c)
3 + 4x
4
9 + x
≤
+ 9¢ +
y
x
x
5
can be translated into a Java expression as:
(3 + 4 * x) / 5 – 10 * (y - 5) * (a + b + c) / x +
9 * (4 / x + (9 + x) / y)
evaluating an expression
operator precedence rule
Though Java has its own way to evaluate an expression behind the scene, the result of
a Java expression and its corresponding arithmetic expression is the same. Therefore, you
can safely apply the arithmetic rule for evaluating a Java expression. Operators contained
within pairs of parentheses are evaluated first. Parentheses can be nested, in which case the
expression in the inner parentheses is evaluated first. When more than one operator is used
in an expression, the following operator precedence rule is used to determine the order of
evaluation.
■
Multiplication, division, and remainder operators are applied first. If an expression
contains several multiplication, division, and remainder operators, they are applied
from left to right.
■
Addition and subtraction operators are applied last. If an expression contains several
addition and subtraction operators, they are applied from left to right.
2.11 Evaluating Expressions and Operator Precedence 51
Here is an example of how an expression is evaluated:
3 + 4 * 4 + 5 * (4 + 3) - 1
(1) inside parentheses first
3 + 4 * 4 + 5 * 7 – 1
(2) multiplication
3 + 16 + 5 * 7 – 1
(3) multiplication
3 + 16 + 35 – 1
(4) addition
19 + 35 – 1
(5) addition
54 – 1
(6) subtraction
53
Listing 2.6 gives a program that converts a Fahrenheit degree to Celsius using the formula
celsius = ( 59 ) (fahrenheit - 32).
LISTING 2.6 FahrenheitToCelsius.java
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
import java.util.Scanner;
public class FahrenheitToCelsius {
public static void main(String[] args) {
Scanner input = new Scanner(System.in);
System.out.print("Enter a degree in Fahrenheit: ");
double fahrenheit = input.nextDouble();
// Convert Fahrenheit to Celsius
double celsius = (5.0 / 9) * (fahrenheit - 32);
System.out.println("Fahrenheit " + fahrenheit + " is " +
celsius + " in Celsius");
divide
}
}
Enter a degree in Fahrenheit: 100
Fahrenheit 100.0 is 37.77777777777778 in Celsius
line#
8
fahrenheit
celsius
100
11
37.77777777777778
Be careful when applying division. Division of two integers yields an integer in Java. 59 is
translated to 5.0 / 9 instead of 5 / 9 in line 11, because 5 / 9 yields 0 in Java.
2.22
How would you write the following arithmetic expression in Java?
3 + d(2 + a)
4
a.
- 9(a + bc) +
3(r + 34)
a + bd
b. 5.5 * (r + 2.5)2.5 + t
integer vs. floating-point
division
✓
Check
Point
52 Chapter 2
Elementary Programming
2.12 Case Study: Displaying the Current Time
Key
Point
VideoNote
Use operators / and %
currentTimeMillis
You can invoke System.currentTimeMillis() to return the current time.
The problem is to develop a program that displays the current time in GMT (Greenwich Mean
Time) in the format hour:minute:second, such as 13:19:8.
The currentTimeMillis method in the System class returns the current time in milliseconds elapsed since midnight, January 1, 1970 GMT, as shown in Figure 2.2. This time
is known as the UNIX epoch. The epoch is the point when time starts, and 1970 was the year
when the UNIX operating system was formally introduced.
UNIX epoch
Elapsed
time
UNIX epoch
01-01-1970
00:00:00 GMT
Time
Current time
System.currentTimeMillis()
FIGURE 2.2 The System.currentTimeMillis() returns the number of milliseconds
since the UNIX epoch.
You can use this method to obtain the current time, and then compute the current second,
minute, and hour as follows.
1. Obtain the total milliseconds since midnight, January 1, 1970, in totalMilliseconds
by invoking System.currentTimeMillis() (e.g., 1203183068328 milliseconds).
2. Obtain the total seconds totalSeconds by dividing totalMilliseconds by 1000
(e.g., 1203183068328 milliseconds / 1000 = 1203183068 seconds).
3. Compute the current second from totalSeconds % 60 (e.g., 1203183068 seconds
% 60 = 8, which is the current second).
4. Obtain the total minutes totalMinutes by dividing totalSeconds by 60 (e.g.,
1203183068 seconds / 60 = 20053051 minutes).
5. Compute the current minute from totalMinutes % 60 (e.g., 20053051 minutes %
60 = 31, which is the current minute).
6. Obtain the total hours totalHours by dividing totalMinutes by 60 (e.g., 20053051
minutes / 60 = 334217 hours).
7. Compute the current hour from totalHours % 24 (e.g., 334217 hours % 24 = 17,
which is the current hour).
Listing 2.7 gives the complete program.
LISTING 2.7 ShowCurrentTime.java
totalMilliseconds
totalSeconds
currentSecond
1
2
3
4
5
6
7
8
9
10
public class ShowCurrentTime {
public static void main(String[] args) {
// Obtain the total milliseconds since midnight, Jan 1, 1970
long totalMilliseconds = System.currentTimeMillis();
// Obtain the total seconds since midnight, Jan 1, 1970
long totalSeconds = totalMilliseconds / 1000;
// Compute the current second in the minute in the hour
long currentSecond = totalSeconds % 60;
2.12 Case Study: Displaying the Current Time 53
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
// Obtain the total minutes
long totalMinutes = totalSeconds / 60;
totalMinutes
// Compute the current minute in the hour
long currentMinute = totalMinutes % 60;
currentMinute
// Obtain the total hours
long totalHours = totalMinutes / 60;
totalHours
// Compute the current hour
long currentHour = totalHours % 24;
currentHour
// Display results
System.out.println("Current time is " + currentHour + ":"
+ currentMinute + ":" + currentSecond + " GMT");
preparing output
}
}
Current time is 17:31:8 GMT
Line 4 invokes System.currentTimeMillis() to obtain the current time in milliseconds as a long value. Thus, all the variables are declared as the long type in this program. The
seconds, minutes, and hours are extracted from the current time using the / and % operators
(lines 6–22).
line#
4
7
10
13
16
19
22
variables
totalMilliseconds
totalSeconds
currentSecond
1203183068328
1203183068
8
totalMinutes
currentMinute
20053051
31
totalHours
334217
currentHour
17
In the sample run, a single digit 8 is displayed for the second. The desirable output
would be 08. This can be fixed by using a method that formats a single digit with a prefix 0
(see Exercise 6.37).
2.23
How do you obtain the current second, minute, and hour?
✓
Check
Point
54 Chapter 2
Elementary Programming
2.13 Augmented Assignment Operators
Key
Point
The operators +, -, *, /, and % can be combined with the assignment operator to form
augmented operators.
Very often the current value of a variable is used, modified, and then reassigned back to the
same variable. For example, the following statement increases the variable count by 1:
count = count + 1;
Java allows you to combine assignment and addition operators using an augmented (or
compound) assignment operator. For example, the preceding statement can be written as
count += 1;
addition assignment operator
The += is called the addition assignment operator. Table 2.4 shows other augmented
assignment operators.
TABLE 2.4 Augmented Assignment Operators
Operator
Name
Example
Equivalent
+=
Addition assignment
i += 8
i = i + 8
-=
Subtraction assignment
i -= 8
i = i – 8
*=
Multiplication assignment
i *= 8
i = i * 8
/=
Division assignment
i /=
i = i / 8
%=
Remainder assignment
i %= 8
8
i = i % 8
The augmented assignment operator is performed last after all the other operators in the
expression are evaluated. For example,
x /= 4 + 5.5 * 1.5;
is same as
x = x / (4 + 5.5 * 1.5);
Caution
There are no spaces in the augmented assignment operators. For example, + = should
be +=.
Note
Like the assignment operator (=), the operators (+=, -=, *=, /=, %=) can be used to
form an assignment statement as well as an expression. For example, in the following
code, x += 2 is a statement in the first line and an expression in the second line.
x += 2; // Statement
System.out.println(x += 2); // Expression
✓
Check
Point
2.24
Show the output of the following code:
double a = 6.5;
a += a + 1;
2.14 Increment and Decrement Operators 55
System.out.println(a);
a = 6;
a /= 2;
System.out.println(a);
2.14 Increment and Decrement Operators
The increment operator (++) and decrement operator (– –) are for incrementing and
decrementing a variable by 1.
The ++ and —— are two shorthand operators for incrementing and decrementing a variable by
1. These are handy because that’s often how much the value needs to be changed in many programming tasks. For example, the following code increments i by 1 and decrements j by 1.
Key
Point
increment operator (++)
decrement operator (−−)
int i = 3, j = 3;
i++; // i becomes 4
j——; // j becomes 2
i++ is pronounced as i plus plus and i—— as i minus minus. These operators are known as
postfix increment (or postincrement) and postfix decrement (or postdecrement), because the
operators ++ and —— are placed after the variable. These operators can also be placed before
the variable. For example,
postincrement
postdecrement
int i = 3, j = 3;
++i; // i becomes 4
——j; // j becomes 2
++i increments i by 1 and ——j decrements j by 1. These operators are known as prefix
increment (or preincrement) and prefix decrement (or predecrement).
As you see, the effect of i++ and ++i or i—— and ——i are the same in the preceding examples. However, their effects are different when they are used in statements that do more than
just increment and decrement. Table 2.5 describes their differences and gives examples.
TABLE 2.5 Increment and Decrement Operators
Operator
Name
Description
Example (assume i = 1)
++var
preincrement
Increment var by 1, and use the
new var value in the statement
int j = ++i;
// j is 2, i is 2
var++
postincrement
Increment var by 1, but use the
original var value in the statement
int j = i++;
// j is 1, i is 2
——var
predecrement
Decrement var by 1, and use the
new var value in the statement
int j = ——i;
// j is 0, i is 0
var——
postdecrement
Decrement var by 1, and use the
original var value in the statement
int j = i——;
// j is 1, i is 0
Here are additional examples to illustrate the differences between the prefix form of ++ (or
——) and the postfix form of ++ (or −−). Consider the following code:
int i = 10;
int newNum = 10 * i++;
Same effect as
System.out.print("i is " + i
+ ", newNum is " + newNum);
i is 11, newNum is 100
int newNum = 10 * i;
i = i + 1;
preincrement
predecrement
56 Chapter 2
Elementary Programming
In this case, i is incremented by 1, then the old value of i is used in the multiplication. So
newNum becomes 100. If i++ is replaced by ++i as follows,
int i = 10;
int newNum = 10 * (++i);
Same effect as
System.out.print("i is " + i
+ ", newNum is " + newNum);
i = i + 1;
int newNum = 10 * i;
i is 11, newNum is 110
i is incremented by 1, and the new value of i is used in the multiplication. Thus newNum
becomes 110.
Here is another example:
double x = 1.0;
double y = 5.0;
double z = x–– + (++y);
After all three lines are executed, y becomes 6.0, z becomes 7.0, and x becomes 0.0.
Tip
Using increment and decrement operators makes expressions short, but it also
makes them complex and difficult to read. Avoid using these operators in expressions that modify multiple variables or the same variable multiple times, such as this
one: int k = ++i + i.
✓
Check
Point
2.25
Which of these statements are true?
a. Any expression can be used as a statement.
b. The expression x++ can be used as a statement.
c. The statement x = x + 5 is also an expression.
d. The statement x = y = x = 0 is illegal.
2.26
Show the output of the following code:
int a = 6;
int b = a++;
System.out.println(a);
System.out.println(b);
a = 6;
b = ++a;
System.out.println(a);
System.out.println(b);
2.15 Numeric Type Conversions
Key
Point
Floating-point numbers can be converted into integers using explicit casting.
Can you perform binary operations with two operands of different types? Yes. If an integer
and a floating-point number are involved in a binary operation, Java automatically converts
the integer to a floating-point value. So, 3 * 4.5 is same as 3.0 * 4.5.
2.15 Numeric Type Conversions 57
You can always assign a value to a numeric variable whose type supports a larger range of
values; thus, for instance, you can assign a long value to a float variable. You cannot, however,
assign a value to a variable of a type with a smaller range unless you use type casting. Casting is
an operation that converts a value of one data type into a value of another data type. Casting a type
with a small range to a type with a larger range is known as widening a type. Casting a type with
a large range to a type with a smaller range is known as narrowing a type. Java will automatically
widen a type, but you must narrow a type explicitly.
The syntax for casting a type is to specify the target type in parentheses, followed by the
variable’s name or the value to be cast. For example, the following statement
casting
widening a type
narrowing a type
System.out.println((int)1.7);
displays 1. When a double value is cast into an int value, the fractional part is truncated.
The following statement
System.out.println((double)1 / 2);
displays 0.5, because 1 is cast to 1.0 first, then 1.0 is divided by 2. However, the statement
System.out.println(1 / 2);
displays 0, because 1 and 2 are both integers and the resulting value should also be an integer.
Caution
Casting is necessary if you are assigning a value to a variable of a smaller type range,
such as assigning a double value to an int variable. A compile error will occur if casting is not used in situations of this kind. However, be careful when using casting, as loss
of information might lead to inaccurate results.
possible loss of precision
Note
Casting does not change the variable being cast. For example, d is not changed after
casting in the following code:
double d = 4.5;
int i = (int)d;
// i becomes 4, but d is still 4.5
Note
In Java, an augmented expression of the form x1 op= x2 is implemented as x1 =
(T)(x1 op x2), where T is the type for x1. Therefore, the following code is correct.
int sum = 0;
sum += 4.5; // sum becomes 4 after this statement
sum += 4.5 is equivalent to sum = (int)(sum + 4.5).
Note
To assign a variable of the int type to a variable of the short or byte type, explicit
casting must be used. For example, the following statements have a compile error:
int i = 1;
byte b = i; // Error because explicit casting is required
However, so long as the integer literal is within the permissible range of the target variable, explicit casting is not needed to assign an integer literal to a variable of the short
or byte type (see Section 2.10, Numeric Literals).
The program in Listing 2.8 displays the sales tax with two digits after the decimal point.
casting in an augmented
expression
58 Chapter 2
Elementary Programming
LISTING 2.8 SalesTax.java
1
2
3
4
5
6
7
8
9
10
11
12
13
casting
import java.util.Scanner;
public class SalesTax {
public static void main(String[] args) {
Scanner input = new Scanner(System.in);
System.out.print("Enter purchase amount: ");
double purchaseAmount = input.nextDouble();
double tax = purchaseAmount * 0.06;
System.out.println("Sales tax is $" + (int)(tax * 100) / 100.0);
}
}
Enter purchase amount: 197.55
Sales tax is $11.85
line#
8
purchaseAmount
tax
output
197.55
10
11.853
11
11.85
The variable purchaseAmount is 197.55 (line 8). The sales tax is 6% of the purchase, so
the tax is evaluated as 11.853 (line 10). Note that
formatting numbers
tax * 100 is 1185.3
(int)(tax * 100) is 1185
(int)(tax * 100) / 100.0 is 11.85
So, the statement in line 11 displays the tax 11.85 with two digits after the decimal point.
✓
Check
Point
2.27
2.28
2.29
Can different types of numeric values be used together in a computation?
What does an explicit casting from a double to an int do with the fractional part of
the double value? Does casting change the variable being cast?
Show the following output:
float f = 12.5F;
int i = (int)f;
System.out.println("f is " + f);
System.out.println("i is " + i);
2.30
2.31
If you change (int)(tax * 100) / 100.0 to (int)(tax * 100) / 100 in line
11 in Listing 2.8, what will be the output for the input purchase amount of 197.55?
Show the output of the following code:
double amount = 5;
System.out.println(amount / 2);
System.out.println(5 / 2);
2.16 Software Development Process 59
2.16 Software Development Process
The software development life cycle is a multistage process that includes requirements
specification, analysis, design, implementation, testing, deployment, and maintenance.
Developing a software product is an engineering process. Software products, no matter how
large or how small, have the same life cycle: requirements specification, analysis, design,
implementation, testing, deployment, and maintenance, as shown in Figure 2.3.
Key
Point
VideoNote
Software development process
Requirements
Specification
Input, Process, Output
IPO
System Analysis
System
Design
Implementation
Testing
Deployment
Maintenance
FIGURE 2.3 At any stage of the software development life cycle, it may be necessary to go
back to a previous stage to correct errors or deal with other issues that might prevent the
software from functioning as expected.
Requirements specification is a formal process that seeks to understand the problem that
the software will address and to document in detail what the software system needs to do.
This phase involves close interaction between users and developers. Most of the examples
in this book are simple, and their requirements are clearly stated. In the real world, however,
problems are not always well defined. Developers need to work closely with their customers
(the individuals or organizations that will use the software) and study the problem carefully to
identify what the software needs to do.
System analysis seeks to analyze the data flow and to identify the system’s input and output. When you do analysis, it helps to identify what the output is first, and then figure out what
input data you need in order to produce the output.
System design is to design a process for obtaining the output from the input. This phase
involves the use of many levels of abstraction to break down the problem into manageable
components and design strategies for implementing each component. You can view each
component as a subsystem that performs a specific function of the system. The essence of
system analysis and design is input, process, and output (IPO).
Implementation involves translating the system design into programs. Separate programs
are written for each component and then integrated to work together. This phase requires
the use of a programming language such as Java. The implementation involves coding, selftesting, and debugging (that is, finding errors, called bugs, in the code).
requirements specification
system analysis
system design
IPO
implementation
60 Chapter 2
testing
deployment
maintenance
VideoNote
Compute loan payments
Elementary Programming
Testing ensures that the code meets the requirements specification and weeds out bugs. An
independent team of software engineers not involved in the design and implementation of the
product usually conducts such testing.
Deployment makes the software available for use. Depending on the type of software,
it may be installed on each user’s machine or installed on a server accessible on the
Internet.
Maintenance is concerned with updating and improving the product. A software
product must continue to perform and improve in an ever-evolving environment. This
requires periodic upgrades of the product to fix newly discovered bugs and incorporate
changes.
To see the software development process in action, we will now create a program that
computes loan payments. The loan can be a car loan, a student loan, or a home mortgage loan.
For an introductory programming course, we focus on requirements specification, analysis,
design, implementation, and testing.
Stage 1: Requirements Specification
The program must satisfy the following requirements:
■
It must let the user enter the interest rate, the loan amount, and the number of years
for which payments will be made.
■
It must compute and display the monthly payment and total payment amounts.
Stage 2: System Analysis
The output is the monthly payment and total payment, which can be obtained using the following formulas:
monthlyPayment =
loanAmount * monthlyInterestRate
1
1 (1 + monthlyInterestRate)numberOfYears * 12
totalPayment = monthlyPayment * numberOfYears * 12
So, the input needed for the program is the monthly interest rate, the length of the loan in
years, and the loan amount.
Note
The requirements specification says that the user must enter the annual interest rate, the
loan amount, and the number of years for which payments will be made. During analysis, however, it is possible that you may discover that input is not sufficient or that some
values are unnecessary for the output. If this happens, you can go back and modify the
requirements specification.
Note
In the real world, you will work with customers from all walks of life. You may develop
software for chemists, physicists, engineers, economists, and psychologists, and of
course you will not have (or need) complete knowledge of all these fields. Therefore,
you don’t have to know how formulas are derived, but given the monthly interest rate,
the number of years, and the loan amount, you can compute the monthly payment in
this program. You will, however, need to communicate with customers and understand
how a mathematical model works for the system.
2.16 Software Development Process 61
Stage 3: System Design
During system design, you identify the steps in the program.
Step 1.
Prompt the user to enter the annual interest rate, the number of years, and the
loan amount.
(The interest rate is commonly expressed as a percentage of the principal for a period of
one year. This is known as the annual interest rate.)
Step 2.
The input for the annual interest rate is a number in percent format, such as
4.5%. The program needs to convert it into a decimal by dividing it by 100. To
obtain the monthly interest rate from the annual interest rate, divide it by 12,
since a year has 12 months. So, to obtain the monthly interest rate in decimal
format, you need to divide the annual interest rate in percentage by 1200. For
example, if the annual interest rate is 4.5%, then the monthly interest rate is
4.5/1200 = 0.00375.
Step 3.
Compute the monthly payment using the preceding formula.
Step 4.
Compute the total payment, which is the monthly payment multiplied by 12 and
multiplied by the number of years.
Step 5.
Display the monthly payment and total payment.
Stage 4: Implementation
Implementation is also known as coding (writing the code). In the formula, you have to compute (1 + monthlyInterestRate)numberOfYears * 12, which can be obtained using Math.pow(1 +
monthlyInterestRate, numberOfYears * 12).
Listing 2.9 gives the complete program.
Math.pow(a, b) method
LISTING 2.9 ComputeLoan.java
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
import java.util.Scanner;
import class
public class ComputeLoan {
public static void main(String[] args) {
// Create a Scanner
Scanner input = new Scanner(System.in);
create a Scanner
// Enter annual interest rate in percentage, e.g., 7.25%
System.out.print("Enter annual interest rate, e.g., 7.25%: ");
double annualInterestRate = input.nextDouble();
enter interest rate
// Obtain monthly interest rate
double monthlyInterestRate = annualInterestRate / 1200;
// Enter number of years
System.out.print(
"Enter number of years as an integer, e.g., 5: ");
int numberOfYears = input.nextInt();
enter years
// Enter loan amount
System.out.print("Enter loan amount, e.g., 120000.95: ");
double loanAmount = input.nextDouble();
enter loan amount
// Calculate payment
double monthlyPayment = loanAmount * monthlyInterestRate / (1
- 1 / Math.pow(1 + monthlyInterestRate, numberOfYears * 12));
monthlyPayment
62 Chapter 2
Elementary Programming
27
28
29
30
31
32
33
34
35
totalPayment
casting
casting
double totalPayment = monthlyPayment * numberOfYears * 12;
// Display results
System.out.println("The monthly payment is $" +
(int)(monthlyPayment * 100) / 100.0);
System.out.println("The total payment is $" +
(int)(totalPayment * 100) / 100.0);
}
}
Enter annual interest rate, e.g., 5.75%: 5.75
Enter number of years as an integer, e.g., 5: 15
Enter loan amount, e.g., 120000.95: 250000
The monthly payment is $2076.02
The total payment is $373684.53
line#
10
13
18
22
25
27
variables
annualInterestRate
5.75
monthlyInterestRate
0.0047916666666
numberOfYears
15
loanAmount
250000
monthlyPayment
2076.0252175
totalPayment
java.lang package
373684.539
Line 10 reads the annual interest rate, which is converted into the monthly interest rate in
line 13.
Choose the most appropriate data type for the variable. For example, numberOfYears is
best declared as an int (line 18), although it could be declared as a long, float, or double.
Note that byte might be the most appropriate for numberOfYears. For simplicity, however,
the examples in this book will use int for integer and double for floating-point values.
The formula for computing the monthly payment is translated into Java code in lines 25–27.
Casting is used in lines 31 and 33 to obtain a new monthlyPayment and totalPayment
with two digits after the decimal points.
The program uses the Scanner class, imported in line 1. The program also uses the Math
class, and you might be wondering why that class isn’t imported into the program. The Math
class is in the java.lang package, and all classes in the java.lang package are implicitly
imported. Therefore, you don’t need to explicitly import the Math class.
Stage 5: Testing
After the program is implemented, test it with some sample input data and verify whether
the output is correct. Some of the problems may involve many cases, as you will see in later
chapters. For these types of problems, you need to design test data that cover all cases.
Tip
incremental code and test
The system design phase in this example identified several steps. It is a good approach
to code and test these steps incrementally by adding them one at a time. This approach
makes it much easier to pinpoint problems and debug the program.
2.17 Case Study: Counting Monetary Units 63
2.32
How would you write the following arithmetic expression?
-b + 2b2 - 4ac
2a
✓
Check
Point
2.17 Case Study: Counting Monetary Units
This section presents a program that breaks a large amount of money into smaller
units.
Key
Point
Suppose you want to develop a program that changes a given amount of money into smaller
monetary units. The program lets the user enter an amount as a double value representing a
total in dollars and cents, and outputs a report listing the monetary equivalent in the maximum
number of dollars, quarters, dimes, nickels, and pennies, in this order, to result in the minimum number of coins.
Here are the steps in developing the program:
1. Prompt the user to enter the amount as a decimal number, such as 11.56.
2. Convert the amount (e.g., 11.56) into cents (1156).
3. Divide the cents by 100 to find the number of dollars. Obtain the remaining cents using
the cents remainder 100.
4. Divide the remaining cents by 25 to find the number of quarters. Obtain the remaining
cents using the remaining cents remainder 25.
5. Divide the remaining cents by 10 to find the number of dimes. Obtain the remaining
cents using the remaining cents remainder 10.
6. Divide the remaining cents by 5 to find the number of nickels. Obtain the remaining
cents using the remaining cents remainder 5.
7. The remaining cents are the pennies.
8. Display the result.
The complete program is given in Listing 2.10.
LISTING 2.10 ComputeChange.java
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
import java.util.Scanner;
import class
public class ComputeChange {
public static void main(String[] args) {
// Create a Scanner
Scanner input = new Scanner(System.in);
// Receive the amount
System.out.print(
"Enter an amount in double, for example 11.56: ");
double amount = input.nextDouble();
enter input
int remainingAmount = (int)(amount * 100);
// Find the number of one dollars
int numberOfOneDollars = remainingAmount / 100;
remainingAmount = remainingAmount % 100;
// Find the number of quarters in the remaining amount
int numberOfQuarters = remainingAmount / 25;
dollars
quarters
64 Chapter 2
dimes
nickels
pennies
output
Elementary Programming
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
remainingAmount = remainingAmount % 25;
// Find the number of dimes in the remaining amount
int numberOfDimes = remainingAmount / 10;
remainingAmount = remainingAmount % 10;
// Find the number of nickels in the remaining amount
int numberOfNickels = remainingAmount / 5;
remainingAmount = remainingAmount % 5;
// Find the number of pennies in the remaining amount
int numberOfPennies = remainingAmount;
// Display results
System.out.println("Your amount " + amount + " consists of");
System.out.println("
" + numberOfOneDollars + " dollars");
System.out.println("
" + numberOfQuarters + " quarters ");
System.out.println("
" + numberOfDimes + " dimes");
System.out.println("
" + numberOfNickels + " nickels");
System.out.println("
" + numberOfPennies + " pennies");
}
}
Enter an amount, for example, 11.56: 11.56
Your amount 11.56 consists of
11 dollars
2 quarters
0 dimes
1 nickels
1 pennies
line#
11
13
16
17
20
21
24
25
28
29
32
variables
amount
remainingAmount
numberOfOneDollars
numberOfQuarters
numberOfDimes
numberOfNickels
numberOfPennies
11.56
1156
56
6
6
1
11
2
0
1
1
The variable amount stores the amount entered from the console (line 11). This variable
is not changed, because the amount has to be used at the end of the program to display the
results. The program introduces the variable remainingAmount (line 13) to store the changing remaining amount.
The variable amount is a double decimal representing dollars and cents. It is converted to
an int variable remainingAmount, which represents all the cents. For instance, if amount
2.18 Common Errors and Pitfalls 65
is 11.56, then the initial remainingAmount is 1156. The division operator yields the integer part of the division, so 1156 / 100 is 11. The remainder operator obtains the remainder
of the division, so 1156 % 100 is 56.
The program extracts the maximum number of singles from the remaining amount and
obtains a new remaining amount in the variable remainingAmount (lines 16–17). It then
extracts the maximum number of quarters from remainingAmount and obtains a new
remainingAmount (lines 20–21). Continuing the same process, the program finds the maximum number of dimes, nickels, and pennies in the remaining amount.
One serious problem with this example is the possible loss of precision when casting a
double amount to an int remainingAmount. This could lead to an inaccurate result. If you
try to enter the amount 10.03, 10.03 * 100 becomes 1002.9999999999999. You will
find that the program displays 10 dollars and 2 pennies. To fix the problem, enter the amount
as an integer value representing cents (see Programming Exercise 2.22).
2.33
Show the output with the input value 1.99.
2.18 Common Errors and Pitfalls
Common elementary programming errors often involve undeclared variables, uninitialized variables, integer overflow, unintended integer division, and round-off errors.
loss of precision
✓
Check
Point
Key
Point
Common Error 1: Undeclared/Uninitialized Variables and Unused Variables
A variable must be declared with a type and assigned a value before using it. A common error
is not declaring a variable or initializing a variable. Consider the following code:
double interestRate = 0.05;
double interest = interestrate * 45;
This code is wrong, because interestRate is assigned a value 0.05; but interestrate
has not been declared and initialized. Java is case sensitive, so it considers interestRate
and interestrate to be two different variables.
If a variable is declared, but not used in the program, it might be a potential programming
error. So, you should remove the unused variable from your program. For example, in the following code, taxRate is never used. It should be removed from the code.
double interestRate = 0.05;
double taxRate = 0.05;
double interest = interestRate * 45;
System.out.println("Interest is " + interest);
If you use an IDE such as Eclipse and NetBeans, you will receive a warning on unused
variables.
Common Error 2: Integer Overflow
Numbers are stored with a limited numbers of digits. When a variable is assigned a value that
is too large (in size) to be stored, it causes overflow. For example, executing the following
statement causes overflow, because the largest value that can be stored in a variable of the int
type is 2147483647. 2147483648 will be too large for an int value.
int value = 2147483647 + 1;
// value will actually be -2147483648
Likewise, executing the following statement causes overflow, because the smallest value that
can be stored in a variable of the int type is -2147483648. -2147483649 is too large in size
to be stored in an int variable.
what is overflow?
66 Chapter 2
Elementary Programming
int value = -2147483648 - 1;
// value will actually be 2147483647
what is underflow?
Java does not report warnings or errors on overflow, so be careful when working with numbers close to the maximum or minimum range of a given type.
When a floating-point number is too small (i.e., too close to zero) to be stored, it causes
underflow. Java approximates it to zero, so normally you don’t need to be concerned about
underflow.
Common Error 3: Round-off Errors
floating-point approximation
A round-off error, also called a rounding error, is the difference between the calculated
approximation of a number and its exact mathematical value. For example, 1/3 is approximately 0.333 if you keep three decimal places, and is 0.3333333 if you keep seven decimal
places. Since the number of digits that can be stored in a variable is limited, round-off errors
are inevitable. Calculations involving floating-point numbers are approximated because these
numbers are not stored with complete accuracy. For example,
System.out.println(1.0 - 0.1 - 0.1 - 0.1 - 0.1 - 0.1);
displays 0.5000000000000001, not 0.5, and
System.out.println(1.0 - 0.9);
displays 0.09999999999999998, not 0.1. Integers are stored precisely. Therefore, calculations with integers yield a precise integer result.
Common Error 4: Unintended Integer Division
Java uses the same divide operator, namely /, to perform both integer and floating-point
division. When two operands are integers, the / operator performs an integer division. The
result of the operation is an integer. The fractional part is truncated. To force two integers to
perform a floating-point division, make one of the integers into a floating-point number. For
example, the code in (a) displays that average is 1 and the code in (b) displays that average
is 1.5.
int number1 = 1;
int number2 = 2;
double average = (number1 + number2) / 2;
System.out.println(average);
int number1 = 1;
int number2 = 2;
double average = (number1 + number2) / 2.0;
System.out.println(average);
(a)
(b)
Common Pitfall 1: Redundant Input Objects
New programmers often write the code to create multiple input objects for each input. For
example, the following code reads an integer and a double value.
Scanner input = new Scanner(System.in);
System.out.print("Enter an integer: ");
int v1 = input.nextInt();
Scanner input1 = new Scanner(System.in);
System.out.print("Enter a double value: ");
double v2 = input1.nextDouble();
BAD CODE
Chapter Summary 67
The code is not wrong, but inefficient. It creates two input objects unnecessarily and may
lead to some subtle errors. You should rewrite the code as follows:
GOOD CODE
Scanner input = new Scanner(System.in);
System.out.print("Enter an integer: ");
int v1 = input.nextInt();
System.out.print("Enter a double value: ");
double v2 = input.nextDouble();
2.34
2.35
2.36
2.37
Can you declare a variable as int and later redeclare it as double?
What is an integer overflow? Can floating-point operations cause overflow?
Will overflow cause a runtime error?
What is a round-off error? Can integer operations cause round-off errors? Can
floating-point operations cause round-off errors?
KEY TERMS
algorithm 34
assignment operator (=) 42
assignment statement 42
byte type 45
casting 57
constant 43
data type 35
declare variables 35
decrement operator (– –) 55
double type 45
expression 42
final keyword 43
float type 45
floating-point number 35
identifier 39
increment operator (++) 55
incremental code and testing 62
int type 45
IPO 39
literal 48
long type 45
narrowing (of types) 57
operands 46
operator 46
overflow 65
postdecrement 55
postincrement 55
predecrement 55
preincrement 55
primitive data type 35
pseudocode 34
requirements specification
scope of a variable 41
short type 45
specific import 38
system analysis 59
system design 59
underflow 66
UNIX epoch 52
variable 35
widening (of types) 57
wildcard import 00
59
CHAPTER SUMMARY
1. Identifiers are names for naming elements such as variables, constants, methods, classes,
packages in a program.
2. An identifier is a sequence of characters that consists of letters, digits, underscores (_),
and dollar signs ($). An identifier must start with a letter or an underscore. It cannot start
with a digit. An identifier cannot be a reserved word. An identifier can be of any length.
3. Variables are used to store data in a program. To declare a variable is to tell the compiler
what type of data a variable can hold.
✓
Check
Point
68 Chapter 2
Elementary Programming
4. There are two types of import statements: specific import and wildcard import. The
specific import specifies a single class in the import statement; the wildcard import
imports all the classes in a package.
5. In Java, the equal sign (=) is used as the assignment operator.
6. A variable declared in a method must be assigned a value before it can be used.
7. A named constant (or simply a constant) represents permanent data that never changes.
8. A named constant is declared by using the keyword final.
9. Java provides four integer types (byte, short, int, and long) that represent integers
of four different sizes.
10. Java provides two floating-point types (float and
double) that represent floating-
point numbers of two different precisions.
11. Java provides operators that perform numeric operations: + (addition), – (subtraction),
* (multiplication), / (division), and % (remainder).
12. Integer arithmetic (/) yields an integer result.
13. The numeric operators in a Java expression are applied the same way as in an arithmetic
expression.
14. Java provides the augmented assignment operators += (addition assignment), –= (subtraction assignment), *= (multiplication assignment), /= (division assignment), and %=
(remainder assignment).
15. The increment operator (++) and the decrement operator (––) increment or decrement
a variable by 1.
16. When evaluating an expression with values of mixed types, Java automatically converts
the operands to appropriate types.
17. You can explicitly convert a value from one type to another using the (type)value
notation.
18. Casting a variable of a type with a small range to a variable of a type with a larger range
is known as widening a type.
19. Casting a variable of a type with a large range to a variable of a type with a smaller range
is known as narrowing a type.
20. Widening a type can be performed automatically without explicit casting. Narrowing a
type must be performed explicitly.
21. In computer science, midnight of January 1, 1970, is known as the UNIX epoch.
QUIZ
Answer the quiz for this chapter online at www.cs.armstrong.edu/liang/intro10e/quiz.html.
Programming Exercises 69
PROGRAMMING EXERCISES
Debugging TIP
The compiler usually gives a reason for a syntax error. If you don’t know how to correct it,
compare your program closely, character by character, with similar examples in the text.
learn from examples
Pedagogical Note
Instructors may ask you to document your analysis and design for selected exercises. Use
your own words to analyze the problem, including the input, output, and what needs to be
computed, and describe how to solve the problem in pseudocode.
Sections 2.2–2.12
2.1
(Convert Celsius to Fahrenheit) Write a program that reads a Celsius degree in
a double value from the console, then converts it to Fahrenheit and displays the
result. The formula for the conversion is as follows:
fahrenheit = (9 / 5) * celsius + 32
Hint: In Java, 9 / 5 is 1, but 9.0 / 5 is 1.8.
Here is a sample run:
Enter a degree in Celsius: 43
43 Celsius is 109.4 Fahrenheit
2.2
(Compute the volume of a cylinder) Write a program that reads in the radius
and length of a cylinder and computes the area and volume using the following
formulas:
area = radius * radius * p
volume = area * length
Here is a sample run:
Enter the radius and length of a cylinder: 5.5 12
The area is 95.0331
The volume is 1140.4
2.3
(Convert feet into meters) Write a program that reads a number in feet, converts it
to meters, and displays the result. One foot is 0.305 meter. Here is a sample run:
Enter a value for feet: 16.5
16.5 feet is 5.0325 meters
document analysis and design
70 Chapter 2
Elementary Programming
2.4
(Convert pounds into kilograms) Write a program that converts pounds into kilograms. The program prompts the user to enter a number in pounds, converts it
to kilograms, and displays the result. One pound is 0.454 kilograms. Here is a
sample run:
Enter a number in pounds: 55.5
55.5 pounds is 25.197 kilograms
*2.5
(Financial application: calculate tips) Write a program that reads the subtotal
and the gratuity rate, then computes the gratuity and total. For example, if the
user enters 10 for subtotal and 15% for gratuity rate, the program displays $1.5
as gratuity and $11.5 as total. Here is a sample run:
Enter the subtotal and a gratuity rate: 10 15
The gratuity is $1.5 and total is $11.5
**2.6
(Sum the digits in an integer) Write a program that reads an integer between 0 and
1000 and adds all the digits in the integer. For example, if an integer is 932, the
sum of all its digits is 14.
Hint: Use the % operator to extract digits, and use the / operator to remove the
extracted digit. For instance, 932 % 10 = 2 and 932 / 10 = 93.
Here is a sample run:
Enter a number between 0 and 1000: 999
The sum of the digits is 27
*2.7
(Find the number of years) Write a program that prompts the user to enter the
minutes (e.g., 1 billion), and displays the number of years and days for the minutes. For simplicity, assume a year has 365 days. Here is a sample run:
Enter the number of minutes: 1000000000
1000000000 minutes is approximately 1902 years and 214 days
*2.8
(Current time) Listing 2.7, ShowCurrentTime.java, gives a program that displays
the current time in GMT. Revise the program so that it prompts the user to enter
the time zone offset to GMT and displays the time in the specified time zone. Here
is a sample run:
Enter the time zone offset to GMT: −5
The current time is 4:50:34
Programming Exercises 71
2.9
(Physics: acceleration) Average acceleration is defined as the change of velocity
divided by the time taken to make the change, as shown in the following formula:
v1 - v0
t
Write a program that prompts the user to enter the starting velocity v0 in meters/
second, the ending velocity v1 in meters/second, and the time span t in seconds,
and displays the average acceleration. Here is a sample run:
a =
Enter v0, v1, and t: 5.5 50.9 4.5
The average acceleration is 10.0889
2.10
(Science: calculating energy) Write a program that calculates the energy needed
to heat water from an initial temperature to a final temperature. Your program
should prompt the user to enter the amount of water in kilograms and the initial
and final temperatures of the water. The formula to compute the energy is
Q = M * (finalTemperature – initialTemperature) * 4184
where M is the weight of water in kilograms, temperatures are in degrees Celsius,
and energy Q is measured in joules. Here is a sample run:
Enter the amount of water in kilograms: 55.5
Enter the initial temperature: 3.5
Enter the final temperature: 10.5
The energy needed is 1625484.0
2.11
(Population projection) Rewrite Programming Exercise 1.11 to prompt the user
to enter the number of years and displays the population after the number of years.
Use the hint in Programming Exercise 1.11 for this program. The population
should be cast into an integer. Here is a sample run of the program:
Enter the number of years: 5
The population in 5 years is 325932970
2.12
(Physics: finding runway length) Given an airplane’s acceleration a and take-off
speed v, you can compute the minimum runway length needed for an airplane to
take off using the following formula:
v2
2a
Write a program that prompts the user to enter v in meters/second (m/s) and the
acceleration a in meters/second squared (m/s2), and displays the minimum runway length. Here is a sample run:
length =
Enter speed and acceleration: 60 3.5
The minimum runway length for this airplane is 514.286
72 Chapter 2
Elementary Programming
**2.13
(Financial application: compound value) Suppose you save $100 each month
into a savings account with the annual interest rate 5%. Thus, the monthly interest rate is 0.05/12 = 0.00417. After the first month, the value in the account
becomes
100 * (1 + 0.00417) = 100.417
After the second month, the value in the account becomes
(100 + 100.417) * (1 + 0.00417) = 201.252
After the third month, the value in the account becomes
(100 + 201.252) * (1 + 0.00417) = 302.507
and so on.
Write a program that prompts the user to enter a monthly saving amount and
displays the account value after the sixth month. (In Exercise 5.30, you will use a
loop to simplify the code and display the account value for any month.)
Enter the monthly saving amount: 100
After the sixth month, the account value is $608.81
*2.14 (Health application: computing BMI) Body Mass Index (BMI) is a measure of
health on weight. It can be calculated by taking your weight in kilograms and
dividing by the square of your height in meters. Write a program that prompts the
user to enter a weight in pounds and height in inches and displays the BMI. Note
that one pound is 0.45359237 kilograms and one inch is 0.0254 meters. Here is
a sample run:
VideoNote
Compute BMI
Enter weight in pounds: 95.5
Enter height in inches: 50
BMI is 26.8573
2.15
(Geometry: distance of two points) Write a program that prompts the user to enter
two points (x1, y1) and (x2, y2) and displays their distance between them.
The formula for computing the distance is 2(x2 - x1)2 + (y2 - y1)2. Note that
you can use Math.pow(a, 0.5) to compute 2a. Here is a sample run:
Enter x1 and y1: 1.5 -3.4
Enter x2 and y2: 4 5
The distance between the two points is 8.764131445842194
2.16
(Geometry: area of a hexagon) Write a program that prompts the user to enter the
side of a hexagon and displays its area. The formula for computing the area of a
hexagon is
Area =
323 2
s,
2
Programming Exercises 73
where s is the length of a side. Here is a sample run:
Enter the side: 5.5
The area of the hexagon is 78.5895
*2.17
(Science: wind-chill temperature) How cold is it outside? The temperature alone
is not enough to provide the answer. Other factors including wind speed, relative humidity, and sunshine play important roles in determining coldness outside.
In 2001, the National Weather Service (NWS) implemented the new wind-chill
temperature to measure the coldness using temperature and wind speed. The
formula is
twc = 35.74 + 0.6215ta - 35.75v 0.16 + 0.4275tav 0.16
where ta is the outside temperature measured in degrees Fahrenheit and v is the
speed measured in miles per hour. twc is the wind-chill temperature. The formula
cannot be used for wind speeds below 2 mph or temperatures below -58 ºF or
above 41ºF.
Write a program that prompts the user to enter a temperature between -58 ºF and
41ºF and a wind speed greater than or equal to 2 and displays the wind-chill temperature. Use Math.pow(a, b) to compute v 0.16. Here is a sample run:
Enter the temperature in Fahrenheit between -58°F and 41°F:
5.3
Enter the wind speed (>=2) in miles per hour: 6
The wind chill index is -5.56707
2.18
(Print a table) Write a program that displays the following table. Cast floatingpoint numbers into integers.
a
1
2
3
4
5
*2.19
b
2
3
4
5
6
pow(a, b)
1
8
81
1024
15625
(Geometry: area of a triangle) Write a program that prompts the user to enter
three points (x1, y1), (x2, y2), (x3, y3) of a triangle and displays its area.
The formula for computing the area of a triangle is
s = (side1 + side2 + side3)/2;
area = 2s(s - side1)(s - side2)(s - side3)
Here is a sample run:
Enter three points for a triangle: 1.5 -3.4 4.6 5 9.5 -3.4
The area of the triangle is 33.6
74 Chapter 2
Elementary Programming
Sections 2.13–2.17
*2.20
(Financial application: calculate interest) If you know the balance and the annual
percentage interest rate, you can compute the interest on the next monthly payment using the following formula:
interest = balance * (annualInterestRate/1200)
Write a program that reads the balance and the annual percentage interest rate and
displays the interest for the next month. Here is a sample run:
Enter balance and interest rate (e.g., 3 for 3%): 1000 3.5
The interest is 2.91667
*2.21
(Financial application: calculate future investment value) Write a program that
reads in investment amount, annual interest rate, and number of years, and displays the future investment value using the following formula:
futureInvestmentValue =
investmentAmount * (1 + monthlyInterestRate)numberOfYears*12
For example, if you enter amount 1000, annual interest rate 3.25%, and number
of years 1, the future investment value is 1032.98.
Here is a sample run:
Enter investment amount: 1000.56
Enter annual interest rate in percentage: 4.25
Enter number of years: 1
Accumulated value is $1043.92
*2.22
*2.23
(Financial application: monetary units) Rewrite Listing 2.10, ComputeChange
.java, to fix the possible loss of accuracy when converting a double value to an
int value. Enter the input as an integer whose last two digits represent the cents.
For example, the input 1156 represents 11 dollars and 56 cents.
(Cost of driving) Write a program that prompts the user to enter the distance to
drive, the fuel efficiency of the car in miles per gallon, and the price per gallon,
and displays the cost of the trip. Here is a sample run:
Enter the driving distance: 900.5
Enter miles per gallon: 25.5
Enter price per gallon: 3.55
The cost of driving is $125.36
CHAPTER
3
SELECTIONS
Objectives
■
To declare boolean variables and write Boolean expressions using
relational operators (§3.2).
■
To implement selection control using one-way if statements (§3.3).
■
To implement selection control using two-way if-else statements
(§3.4).
■
To implement selection control using nested if and multi-way if
statements (§3.5).
■
To avoid common errors and pitfalls in if statements (§3.6).
■
To generate random numbers using the Math.random() method (§3.7).
■
To program using selection statements for a variety of examples
(SubtractionQuiz, BMI, ComputeTax) (§§3.7–3.9).
■
To combine conditions using logical operators (!, &&, ||, and ^)
(§3.10).
■
To program using selection statements with combined conditions
(LeapYear, Lottery) (§§3.11–3.12).
■
To implement selection control using switch statements (§3.13).
■
To write expressions using the conditional expression (§3.14).
■
To examine the rules governing operator precedence and associativity
(§3.15).
■
To apply common techniques to debug errors (§3.16).
76 Chapter 3
Selections
3.1 Introduction
problem
Key
Point
selection statements
The program can decide which statements to execute based on a condition.
If you enter a negative value for radius in Listing 2.2, ComputeAreaWithConsoleInput.java,
the program displays an invalid result. If the radius is negative, you don’t want the program to
compute the area. How can you deal with this situation?
Like all high-level programming languages, Java provides selection statements: statements
that let you choose actions with alternative courses. You can use the following selection statement to replace lines 12–17 in Listing 2.2:
if (radius < 0) {
System.out.println("Incorrect input");
}
else {
area = radius * radius * 3.14159;
System.out.println("Area is " + area);
}
Selection statements use conditions that are Boolean expressions. A Boolean expression is
an expression that evaluates to a Boolean value: true or false. We now introduce Boolean
types and relational operators.
Boolean expression
Boolean value
3.2 boolean Data Type
Key
Point
boolean data type
relational operators
The boolean data type declares a variable with the value either true or false.
How do you compare two values, such as whether a radius is greater than 0, equal to 0, or less
than 0? Java provides six relational operators (also known as comparison operators), shown
in Table 3.1, which can be used to compare two values (assume radius is 5 in the table).
TABLE 3.1 Relational Operators
Java Operator
Mathematics Symbol
Name
Example (radius is 5)
Result
<
<
less than
radius < 0
false
<=
≤
less than or equal to
radius <= 0
false
>
>
greater than
radius > 0
true
>=
≥
greater than or equal to
radius >= 0
true
==
=
equal to
radius == 0
false
!=
≠
not equal to
radius != 0
true
Caution
== vs. =
The equality testing operator is two equal signs (==), not a single equal sign (=). The
latter symbol is for assignment.
The result of the comparison is a Boolean value: true or false. For example, the following statement displays true:
double radius = 1;
System.out.println(radius > 0);
Boolean variable
A variable that holds a Boolean value is known as a Boolean variable. The boolean
data type is used to declare Boolean variables. A boolean variable can hold one of the
3.2 boolean Data Type 77
two values: true or false. For example, the following statement assigns true to the
variable lightsOn:
boolean lightsOn = true;
true and false are literals, just like a number such as 10. They are treated as reserved words
and cannot be used as identifiers in the program.
Suppose you want to develop a program to let a first-grader practice addition. The program
randomly generates two single-digit integers, number1 and number2, and displays to the student
a question such as “What is 1 + 7?,” as shown in the sample run in Listing 3.1. After the student
types the answer, the program displays a message to indicate whether it is true or false.
There are several ways to generate random numbers. For now, generate the first integer
using System.currentTimeMillis() % 10 and the second using System.currentTimeMillis() / 7 % 10. Listing 3.1 gives the program. Lines 5–6 generate two numbers,
number1 and number2. Line 14 obtains an answer from the user. The answer is graded in
line 18 using a Boolean expression number1 + number2 == answer.
Boolean literals
VideoNote
Program addition quiz
LISTING 3.1 AdditionQuiz.java
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
import java.util.Scanner;
public class AdditionQuiz {
public static void main(String[] args) {
int number1 = (int)(System.currentTimeMillis() % 10);
int number2 = (int)(System.currentTimeMillis() / 7 % 10);
generate number1
generate number2
// Create a Scanner
Scanner input = new Scanner(System.in);
System.out.print(
"What is " + number1 + " + " + number2 + "? ");
show question
int number = input.nextInt();
System.out.println(
number1 + " + " + number2 + " = " + answer + " is " +
(number1 + number2 == answer));
}
}
What is 1 + 7? 8
1 + 7 = 8 is true
What is 4 + 8? 9
4 + 8 = 9 is false
line#
5
6
14
16
number1
number2
answer
output
4
8
9
4 + 8 = 9 is false
display result
78 Chapter 3
Selections
✓
Check
Point
3.1
3.2
List six relational operators.
Assuming that x is 1, show the result of the following Boolean expressions:
(x
(x
(x
(x
(x
3.3
> 0)
< 0)
!= 0)
>= 0)
!= 1)
Can the following conversions involving casting be allowed? Write a test program to
verify your answer.
boolean b = true;
i = (int)b;
int i = 1;
boolean b = (boolean)i;
3.3 if Statements
Key
Point
why if statement?
An if statement is a construct that enables a program to specify alternative paths of execution.
The preceding program displays a message such as “6 + 2 = 7 is false.” If you wish the
message to be “6 + 2 = 7 is incorrect,” you have to use a selection statement to make this
minor change.
Java has several types of selection statements: one-way if statements, two-way if-else
statements, nested if statements, multi-way if-else statements, switch statements, and
conditional expressions.
A one-way if statement executes an action if and only if the condition is true. The syntax
for a one-way if statement is:
if (boolean-expression) {
statement(s);
}
if statement
The flowchart in Figure 3.1a illustrates how Java executes the syntax of an if statement.
A flowchart is a diagram that describes an algorithm or process, showing the steps as boxes
of various kinds, and their order by connecting these with arrows. Process operations are
represented in these boxes, and arrows connecting them represent the flow of control. A diamond box denotes a Boolean condition and a rectangle box represents statements.
flowchart
booleanexpression
true
false
(radius >= 0)
false
true
Statement(s)
area = radius * radius * PI;
System.out.println("The area for the circle of" +
" radius " + radius + " is " + area);
(a)
(b)
FIGURE 3.1 An if statement executes statements if the boolean-expression evaluates to true.
3.3 if Statements 79
If the boolean-expression evaluates to true, the statements in the block are executed.
As an example, see the following code:
if (radius >= 0) {
area = radius * radius * PI;
System.out.println("The area for the circle of radius " +
radius + " is " + area);
}
The flowchart of the preceding statement is shown in Figure 3.1b. If the value of radius
is greater than or equal to 0, then the area is computed and the result is displayed; otherwise,
the two statements in the block will not be executed.
The boolean-expression is enclosed in parentheses. For example, the code in (a) is
wrong. It should be corrected, as shown in (b).
if i > 0 {
System.out.println("i is positive");
}
if (i > 0) {
System.out.println("i is positive");
}
(a) Wrong
(b) Correct
The block braces can be omitted if they enclose a single statement. For example, the following statements are equivalent.
if (i > 0) {
System.out.println("i is positive");
}
Equivalent
if (i > 0)
System.out.println("i is positive");
(a)
(b)
Note
Omitting braces makes the code shorter, but it is prone to errors. It is a common mistake
to forget the braces when you go back to modify the code that omits the braces.
Omitting braces or not
Listing 3.2 gives a program that prompts the user to enter an integer. If the number is a
multiple of 5, the program displays HiFive. If the number is divisible by 2, it displays HiEven.
LISTING 3.2 SimpleIfDemo.java
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
import java.util.Scanner;
public class SimpleIfDemo {
public static void main(String[] args) {
Scanner input = new Scanner(System.in);
System.out.println("Enter an integer: ");
int number = input.nextInt();
}
}
enter input
if (number % 5 == 0)
System.out.println("HiFive");
check 5
if (number % 2 == 0)
System.out.println("HiEven");
check even
80 Chapter 3
Selections
Enter an integer:
HiEven
Enter an integer:
HiFive
HiEven
4
30
The program prompts the user to enter an integer (lines 6–7) and displays HiFive if it is
divisible by 5 (lines 9–10) and HiEven if it is divisible by 2 (lines 12–13).
✓
Check
Point
3.4
3.5
Write an if statement that assigns 1 to x if y is greater than 0.
Write an if statement that increases pay by 3% if score is greater than 90.
3.4 Two-Way if-else Statements
Key
Point
An if-else statement decides the execution path based on whether the condition is
true or false.
A one-way if statement performs an action if the specified condition is true. If the condition
is false, nothing is done. But what if you want to take alternative actions when the condition
is false? You can use a two-way if-else statement. The actions that a two-way if-else
statement specifies differ based on whether the condition is true or false.
Here is the syntax for a two-way if-else statement:
if (boolean-expression) {
statement(s)-for-the-true-case;
}
else {
statement(s)-for-the-false-case;
}
The flowchart of the statement is shown in Figure 3.2.
true
Statement(s) for the true case
booleanexpression
false
Statement(s) for the false case
FIGURE 3.2 An if-else statement executes statements for the true case if the Booleanexpression evaluates to true; otherwise, statements for the false case are executed.
3.5 Nested if and Multi-Way if-else Statements 81
If the boolean-expression evaluates to true, the statement(s) for the true case are
executed; otherwise, the statement(s) for the false case are executed. For example, consider
the following code:
if (radius >= 0) {
area = radius * radius * PI;
System.out.println("The area for the circle of radius " +
radius + " is " + area);
}
else {
System.out.println("Negative input");
}
two-way if-else statement
If radius >= 0 is true, area is computed and displayed; if it is false, the message
"Negative input" is displayed.
As usual, the braces can be omitted if there is only one statement within them. The braces
enclosing the System.out.println("Negative input") statement can therefore be
omitted in the preceding example.
Here is another example of using the if-else statement. The example checks whether a
number is even or odd, as follows:
if (number % 2 == 0)
System.out.println(number + " is even.");
else
System.out.println(number + " is odd.");
3.6
3.7
Write an if statement that increases pay by 3% if score is greater than 90, otherwise increases pay by 1%.
What is the output of the code in (a) and (b) if number is 30? What if number is 35?
if (number % 2 == 0)
System.out.println(number + " is even.");
System.out.println(number + " is odd.");
✓
Check
Point
if (number % 2 == 0)
System.out.println(number + " is even.");
else
System.out.println(number + " is odd.");
(a)
(b)
3.5 Nested if and Multi-Way if-else Statements
An if statement can be inside another if statement to form a nested if statement.
The statement in an if or if-else statement can be any legal Java statement, including
another if or if-else statement. The inner if statement is said to be nested inside the outer
if statement. The inner if statement can contain another if statement; in fact, there is no
limit to the depth of the nesting. For example, the following is a nested if statement:
if (i > k) {
if (j > k)
System.out.println("i and j are greater than k");
}
else
System.out.println("i is less than or equal to k");
The if (j > k) statement is nested inside the if (i > k) statement.
The nested if statement can be used to implement multiple alternatives. The statement
given in Figure 3.3a, for instance, prints a letter grade according to the score, with multiple
alternatives.
Key
Point
nested if statement
82 Chapter 3
Selections
if (score >= 90.0)
System.out.print("A");
else
if (score >= 80.0)
System.out.print("B");
else
if (score >= 70.0)
System.out.print("C");
else
if (score >= 60.0)
System.out.print("D");
else
System.out.print("F");
Equivalent
if (score >= 90.0)
System.out.print("A");
else if (score >= 80.0)
System.out.print("B");
else if (score >= 70.0)
System.out.print("C");
else if (score >= 60.0)
System.out.print("D");
else
System.out.print("F");
This is better
(a)
(b)
FIGURE 3.3 A preferred format for multiple alternatives is shown in (b) using a multi-way
if-else statement.
The execution of this if statement proceeds as shown in Figure 3.4. The first condition
(score >= 90.0) is tested. If it is true, the grade is A. If it is false, the second condition
(score >= 80.0) is tested. If the second condition is true, the grade is B. If that condition
is false, the third condition and the rest of the conditions (if necessary) are tested until a
condition is met or all of the conditions prove to be false. If all of the conditions are false,
the grade is F. Note that a condition is tested only when all of the conditions that come before
it are false.
score >= 90
true
false
score >= 80
false
grade is A
true
score >= 70
false
grade is B
true
score >= 60
false
grade is C
true
grade is D
grade is F
FIGURE 3.4 You can use a multi-way if-else statement to assign a grade.
3.6 Common Errors and Pitfalls 83
The if statement in Figure 3.3a is equivalent to the if statement in Figure 3.3b. In fact,
Figure 3.3b is the preferred coding style for multiple alternative if statements. This style,
called multi-way if-else statements, avoids deep indentation and makes the program easy
to read.
3.8
Suppose x = 3 and y = 2; show the output, if any, of the following code. What is
the output if x = 3 and y = 4? What is the output if x = 2 and y = 2? Draw a
flowchart of the code.
multi-way if statement
✓
Check
Point
if (x > 2) {
if (y > 2) {
z = x + y;
System.out.println("z is " + z);
}
}
else
System.out.println("x is " + x);
3.9
Suppose x = 2 and y = 3. Show the output, if any, of the following code. What is
the output if x = 3 and y = 2? What is the output if x = 3 and y = 3?
if (x > 2)
if (y > 2) {
int z = x + y;
System.out.println("z is " + z);
}
else
System.out.println("x is " + x);
3.10
What is wrong in the following code?
if (score >= 60.0)
System.out.println("D");
else if (score >= 70.0)
System.out.println("C");
else if (score >= 80.0)
System.out.println("B");
else if (score >= 90.0)
System.out.println("A");
else
System.out.println("F");
3.6 Common Errors and Pitfalls
Forgetting necessary braces, ending an if statement in the wrong place, mistaking ==
for =, and dangling else clauses are common errors in selection statements.
Duplicated statements in if-else statements and testing equality of double values
are common pitfalls.
The following errors are common among new programmers.
Common Error 1: Forgetting Necessary Braces
The braces can be omitted if the block contains a single statement. However, forgetting the
braces when they are needed for grouping multiple statements is a common programming
error. If you modify the code by adding new statements in an if statement without braces,
you will have to insert the braces. For example, the following code in (a) is wrong. It should
be written with braces to group multiple statements, as shown in (b).
Key
Point
84 Chapter 3
Selections
if (radius >= 0)
area = radius * radius * PI;
System.out.println("The area "
+ " is " + area);
if (radius >= 0) {
area = radius * radius * PI;
System.out.println("The area "
+ " is " + area);
}
(a) Wrong
(b) Correct
Common Error 2: Wrong Semicolon at the if Line
Adding a semicolon at the end of an if line, as shown in (a) below, is a common mistake.
Logic error
Empty block
if (radius >= 0);
{
area = radius * radius * PI;
System.out.println("The area "
+ " is " + area);
}
Equivalent
if (radius >= 0) { };
{
area = radius * radius * PI;
System.out.println("The area "
+ " is " + area);
}
(a)
(b)
This mistake is hard to find, because it is neither a compile error nor a runtime error; it is a
logic error. The code in (a) is equivalent to that in (b) with an empty block.
This error often occurs when you use the next-line block style. Using the end-of-line block
style can help prevent this error.
Common Error 3: Redundant Testing of Boolean Values
To test whether a boolean variable is true or false in a test condition, it is redundant to
use the equality testing operator like the code in (a):
if (even == true)
System.out.println(
"It is even.");
(a)
Equivalent
This is better
if (even)
System.out.println(
"It is even.");
(b)
Instead, it is better to test the boolean variable directly, as shown in (b). Another good
reason for doing this is to avoid errors that are difficult to detect. Using the = operator instead
of the == operator to compare the equality of two items in a test condition is a common error.
It could lead to the following erroneous statement:
if (even = true)
System.out.println("It is even.");
This statement does not have compile errors. It assigns true to even, so that even is
always true.
Common Error 4: Dangling else Ambiguity
The code in (a) below has two if clauses and one else clause. Which if clause is matched
by the else clause? The indentation indicates that the else clause matches the first if clause.
3.6 Common Errors and Pitfalls 85
However, the else clause actually matches the second if clause. This situation is known as
the dangling else ambiguity. The else clause always matches the most recent unmatched if
clause in the same block. So, the statement in (a) is equivalent to the code in (b).
int i = 1, j = 2, k = 3;
if (i > j)
if (i > k)
System.out.println("A");
else
System.out.println("B");
Equivalent
This is better
with correct
indentation
int i = 1, j = 2, k = 3;
if (i > j)
if (i > k)
System.out.println("A");
else
System.out.println("B");
(a)
(b)
Since (i > j) is false, nothing is displayed from the statements in (a) and (b). To force
the else clause to match the first if clause, you must add a pair of braces:
int i = 1, j = 2, k = 3;
if (i > j) {
if (i > k)
System.out.println("A");
}
else
System.out.println("B");
This statement displays B.
Common Error 5: Equality Test of Two Floating-Point Values
As discussed in Common Error 3 in Section 2.18, floating-point numbers have a limited precision and calculations; involving floating-point numbers can introduce round-off errors. So,
equality test of two floating-point values is not reliable. For example, you expect the following code to display true, but surprisingly it displays false.
double x = 1.0 - 0.1 - 0.1 - 0.1 - 0.1 - 0.1;
System.out.println(x == 0.5);
Here, x is not exactly 0.5, but is 0.5000000000000001. You cannot reliably test equality
of two floating-point values. However, you can compare whether they are close enough by
testing whether the difference of the two numbers is less than some threshold. That is, two
numbers x and y are very close if |x−y| < e for a very small value, e. e, a Greek letter pronounced epsilon, is commonly used to denote a very small value. Normally, you set e to 10-14
for comparing two values of the double type and to 10-7 for comparing two values of the
float type. For example, the following code
final double EPSILON = 1E-14;
double x = 1.0 - 0.1 - 0.1 - 0.1 - 0.1 - 0.1;
if (Math.abs(x - 0.5) < EPSILON)
System.out.println(x + " is approximately 0.5");
will display that
0.5000000000000001 is approximately 0.5
The Math.abs(a) method can be used to return the absolute value of a.
dangling else ambiguity
86 Chapter 3
Selections
Common Pitfall 1: Simplifying Boolean Variable Assignment
Often, new programmers write the code that assigns a test condition to a boolean variable
like the code in (a):
if (number % 2 == 0)
even = true;
else
even = false;
Equivalent
boolean even
= number % 2 == 0;
This is shorter
(a)
(b)
This is not an error, but it should be better written as shown in (b).
Common Pitfall 2: Avoiding Duplicate Code in Different Cases
Often, new programmers write the duplicate code in different cases that should be combined
in one place. For example, the highlighted code in the following statement is duplicated.
if (inState) {
tuition = 5000;
System.out.println("The tuition is " + tuition);
}
else {
tuition = 15000;
System.out.println("The tuition is " + tuition);
}
This is not an error, but it should be better written as follows:
if (inState) {
tuition = 5000;
}
else {
tuition = 15000;
}
System.out.println("The tuition is " + tuition);
The new code removes the duplication and makes the code easy to maintain, because you only
need to change in one place if the print statement is modified.
✓
Check
Point
if (i > 0) if
(j > 0)
x = 0; else
if (k > 0) y = 0;
else z = 0;
3.11
Which of the following statements are equivalent? Which ones are correctly
indented?
if (i > 0) {
if (j > 0)
x = 0;
else if (k > 0)
y = 0;
}
else
z = 0;
(b)
(a)
3.12
if (i > 0)
if (j > 0)
x = 0;
else if (k > 0)
y = 0;
else
z = 0;
if (i > 0)
if (j > 0)
x = 0;
else if (k > 0)
y = 0;
else
z = 0;
(c)
Rewrite the following statement using a Boolean expression:
if (count % 10 == 0)
newLine = true;
else
newLine = false;
(d)
3.7 Generating Random Numbers 87
3.13
Are the following statements correct? Which one is better?
if (age < 16)
System.out.println
("Cannot get a driver's license");
if (age >= 16)
System.out.println
("Can get a driver's license");
if (age < 16)
System.out.println
("Cannot get a driver's license");
else
System.out.println
("Can get a driver's license");
(b)
(a)
3.14
What is the output of the following code if number is 14, 15, or 30?
if (number % 2 == 0)
System.out.println
(number + " is even");
if (number % 5 == 0)
System.out.println
(number + " is multiple of 5");
if (number % 2 == 0)
System.out.println
(number + " is even");
else if (number % 5 == 0)
System.out.println
(number + " is multiple of 5");
(a)
(b)
3.7 Generating Random Numbers
You can use Math.random() to obtain a random double value between 0.0 and 1.0,
excluding 1.0.
Suppose you want to develop a program for a first-grader to practice subtraction. The program
randomly generates two single-digit integers, number1 and number2, with number1 >=
number2, and it displays to the student a question such as “What is 9 - 2?” After the student
enters the answer, the program displays a message indicating whether it is correct.
The previous programs generate random numbers using System.currentTimeMillis().
A better approach is to use the random() method in the Math class. Invoking this method
returns a random double value d such that 0.0 … d 6 1.0. Thus, (int)(Math.random() *
10) returns a random single-digit integer (i.e., a number between 0 and 9).
The program can work as follows:
Key
Point
VideoNote
Program subtraction quiz
random() method
1. Generate two single-digit integers into number1 and number2.
2. If number1 < number2, swap number1 with number2.
3. Prompt the student to answer, "What is number1 – number2?"
4. Check the student’s answer and display whether the answer is correct.
The complete program is shown in Listing 3.3.
LISTING 3.3 SubtractionQuiz.java
1
2
3
4
5
6
7
8
9
10
11
import java.util.Scanner;
public class SubtractionQuiz {
public static void main(String[] args) {
// 1. Generate two random single-digit integers
int number1 = (int)(Math.random() * 10);
int number2 = (int)(Math.random() * 10);
// 2. If number1 < number2, swap number1 with number2
if (number1 < number2) {
int temp = number1;
random number
88 Chapter 3
Selections
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
get answer
check the answer
number1 = number2;
number2 = temp;
}
// 3. Prompt the student to answer ”What is number1 – number2?”
System.out.print
("What is " + number1 + " - " + number2 + "? ");
Scanner input = new Scanner(System.in);
int answer = input.nextInt();
// 4. Grade the answer and display the result
if (number1 - number2 == answer)
System.out.println("You are correct!");
else {
System.out.println("Your answer is wrong.");
System.out.println(number1 + " - " + number2 +
" should be " + (number1 - number2));
}
}
}
What is 6 - 6? 0
You are correct!
What is 9 - 2? 5
Your answer is wrong
9 - 2 is 7
line#
6
number1
number2
7
13
answer
output
2
9
11
12
temp
2
9
2
20
5
26
Your answer is wrong
9 – 2 should be 7
To swap two variables number1 and number2, a temporary variable temp (line 11) is used
to first hold the value in number1. The value in number2 is assigned to number1 (line 12),
and the value in temp is assigned to number2 (line 13).
✓
Check
Point
3.15
Which of the following is a possible output from invoking Math.random()?
323.4, 0.5, 34, 1.0, 0.0, 0.234
3.16
a. How do you generate a random integer i such that 0 … i 6 20?
b. How do you generate a random integer i such that 10 … i 6 20?
c. How do you generate a random integer i such that 10 … i … 50?
d. Write an expression that returns 0 or 1 randomly.
3.8 Case Study: Computing Body Mass Index 89
3.8 Case Study: Computing Body Mass Index
You can use nested if statements to write a program that interprets body mass index.
Body Mass Index (BMI) is a measure of health based on height and weight. It can be calculated by taking your weight in kilograms and dividing it by the square of your height in
meters. The interpretation of BMI for people 20 years or older is as follows:
BMI
Interpretation
BMI < 18.5
18.5 ≤ BMI < 25.0
25.0 ≤ BMI < 30.0
30.0 ≤ BMI
Underweight
Normal
Overweight
Obese
Key
Point
Write a program that prompts the user to enter a weight in pounds and height in inches and
displays the BMI. Note that one pound is 0.45359237 kilograms and one inch is 0.0254
meters. Listing 3.4 gives the program.
LISTING 3.4 ComputeAndInterpretBMI.java
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
import java.util.Scanner;
public class ComputeAndInterpretBMI {
public static void main(String[] args) {
Scanner input = new Scanner(System.in);
// Prompt the user to enter weight in pounds
System.out.print("Enter weight in pounds: ");
double weight = input.nextDouble();
input weight
// Prompt the user to enter height in inches
System.out.print("Enter height in inches: ");
double height = input.nextDouble();
input height
final double KILOGRAMS_PER_POUND = 0.45359237; // Constant
final double METERS_PER_INCH = 0.0254; // Constant
// Compute BMI
double weightInKilograms = weight * KILOGRAMS_PER_POUND;
double heightInMeters = height * METERS_PER_INCH;
double bmi = weightInKilograms /
(heightInMeters * heightInMeters);
// Display result
System.out.println("BMI is " + bmi);
if (bmi < 18.5)
System.out.println("Underweight");
else if (bmi < 25)
System.out.println("Normal");
else if (bmi < 30)
System.out.println("Overweight");
else
System.out.println("Obese");
}
}
compute bmi
display output
90 Chapter 3
Selections
Enter weight in pounds: 146
Enter height in inches: 70
BMI is 20.948603801493316
Normal
line#
9
13
weight
height
weightInKilograms
heightInMeters
bmi
output
146
70
19
66.22448602
20
1.778
21
20.9486
25
BMI is
20.95
31
Normal
The constants KILOGRAMS_PER_POUND and METERS_PER_INCH are defined in lines
15–16. Using constants here makes programs easy to read.
You should test the input that covers all possible cases for BMI to ensure that the program
works for all cases.
test all cases
3.9 Case Study: Computing Taxes
Key
Point
VideoNote
Use multi-way if-else
statements
You can use nested if statements to write a program for computing taxes.
The United States federal personal income tax is calculated based on filing status and taxable income. There are four filing statuses: single filers, married filing jointly or qualified
widow(er), married filing separately, and head of household. The tax rates vary every year.
Table 3.2 shows the rates for 2009. If you are, say, single with a taxable income of $10,000,
the first $8,350 is taxed at 10% and the other $1,650 is taxed at 15%, so, your total tax is
$1,082.50.
TABLE 3.2 2009 U.S. Federal Personal Tax Rates
Marginal
Tax Rate
10%
Married Filing Jointly
or Qualifying Widow(er)
Single
$0 – $8,350
$0 – $16,700
Married Filing Separately
$0 – $8,350
Head of Household
$0 – $11,950
15%
$8,351 – $33,950
$16,701 – $67,900
$8,351 – $33,950
$11,951 – $45,500
25%
$33,951 – $82,250
$67,901 – $137,050
$33,951 – $68,525
$45,501 – $117,450
28%
$82,251 – $171,550
$137,051 – $208,850
$68,526 – $104,425
$117,451 – $190,200
33%
$171,551 – $372,950
$208,851 – $372,950
$104,426 – $186,475
$190,201 – $372,950
35%
$372,951+
$372,951+
$186,476+
$372,951+
You are to write a program to compute personal income tax. Your program should prompt
the user to enter the filing status and taxable income and compute the tax. Enter 0 for single
filers, 1 for married filing jointly or qualified widow(er), 2 for married filing separately, and
3 for head of household.
3.9 Case Study: Computing Taxes 91
Your program computes the tax for the taxable income based on the filing status. The filing
status can be determined using if statements outlined as follows:
if (status == 0) {
// Compute tax for single filers
}
else if (status == 1) {
// Compute tax for married filing jointly or qualifying widow(er)
}
else if (status == 2) {
// Compute tax for married filing separately
}
else if (status == 3) {
// Compute tax for head of household
}
else {
// Display wrong status
}
For each filing status there are six tax rates. Each rate is applied to a certain amount of
taxable income. For example, of a taxable income of $400,000 for single filers, $8,350 is
taxed at 10%, (33,950 – 8,350) at 15%, (82,250 – 33,950) at 25%, (171,550 – 82,250) at 28%,
(372,950 – 171,550) at 33%, and (400,000 – 372,950) at 35%.
Listing 3.5 gives the solution for computing taxes for single filers. The complete solution
is left as an exercise.
LISTING 3.5 ComputeTax.java
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
import java.util.Scanner;
public class ComputeTax {
public static void main(String[] args) {
// Create a Scanner
Scanner input = new Scanner(System.in);
// Prompt the user to enter filing status
System.out.print("(0-single filer, 1-married jointly or " +
"qualifying widow(er), 2-married separately, 3-head of " +
"household) Enter the filing status: ");
int status = input.nextInt();
input status
// Prompt the user to enter taxable income
System.out.print("Enter the taxable income: ");
double income = input.nextDouble();
input income
// Compute tax
double tax = 0;
compute tax
if (status == 0) { // Compute tax for single filers
if (income <= 8350)
tax = income * 0.10;
else if (income <= 33950)
tax = 8350 * 0.10 + (income - 8350) * 0.15;
else if (income <= 82250)
tax = 8350 * 0.10 + (33950 - 8350) * 0.15 +
(income - 33950) * 0.25;
else if (income <= 171550)
tax = 8350 * 0.10 + (33950 - 8350) * 0.15 +
(82250 - 33950) * 0.25 + (income - 82250) * 0.28;
92 Chapter 3
Selections
exit program
display output
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
else if (income <= 372950)
tax = 8350 * 0.10 + (33950 - 8350) *
(82250 - 33950) * 0.25 + (171550 (income - 171550) * 0.33;
else
tax = 8350 * 0.10 + (33950 - 8350) *
(82250 - 33950) * 0.25 + (171550 (372950 - 171550) * 0.33 + (income
0.15 +
82250) * 0.28 +
0.15 +
82250) * 0.28 +
- 372950) * 0.35;
}
else if (status == 1) { // Left as an exercise
// Compute tax for married file jointly or qualifying widow(er)
}
else if (status == 2) { // Compute tax for married separately
// Left as an exercise
}
else if (status == 3) { // Compute tax for head of household
// Left as an exercise
}
else {
System.out.println("Error: invalid status");
System.exit(1);
}
// Display the result
System.out.println("Tax is " + (int)(tax * 100) / 100.0);
}
}
(0-single filer, 1-married jointly or qualifying widow(er),
2-married separately, 3-head of household)
Enter the filing status: 0
Enter the taxable income: 400000
Tax is 117683.5
line#
13
17
income
tax
output
0
400000
20
0
38
117683.5
57
System.exit(status)
status
Tax is 117683.5
The program receives the filing status and taxable income. The multi-way if-else statements (lines 22, 42, 45, 48, 51) check the filing status and compute the tax based on the filing
status.
System.exit(status) (line 53) is defined in the System class. Invoking this method
terminates the program. The status 0 indicates that the program is terminated normally. A
nonzero status code indicates abnormal termination.
An initial value of 0 is assigned to tax (line 20). A compile error would occur if it had
no initial value, because all of the other statements that assign values to tax are within the
if statement. The compiler thinks that these statements may not be executed and therefore
reports a compile error.
3.10 Logical Operators 93
To test a program, you should provide the input that covers all cases. For this program,
your input should cover all statuses (0, 1, 2, 3). For each status, test the tax for each of the six
brackets. So, there are a total of 24 cases.
test all cases
Tip
For all programs, you should write a small amount of code and test it before moving on
to add more code. This is called incremental development and testing. This approach
makes testing easier, because the errors are likely in the new code you just added.
3.17
✓
Are the following two statements equivalent?
if (income <= 10000)
tax = income * 0.1;
else if (income <= 20000)
tax = 1000 +
(income – 10000) * 0.15;
incremental development and
testing
Check
Point
if (income <= 10000)
tax = income * 0.1;
else if (income > 10000 &&
income <= 20000)
tax = 1000 +
(income – 10000) * 0.15;
3.10 Logical Operators
The logical operators !, &&, ||, and ^ can be used to create a compound Boolean
expression.
Sometimes, whether a statement is executed is determined by a combination of several conditions. You can use logical operators to combine these conditions to form a compound Boolean
expression. Logical operators, also known as Boolean operators, operate on Boolean values
to create a new Boolean value. Table 3.3 lists the Boolean operators. Table 3.4 defines the
not (!) operator, which negates true to false and false to true. Table 3.5 defines the and
(&&) operator. The and (&&) of two Boolean operands is true if and only if both operands are
true. Table 3.6 defines the or (||) operator. The or (||) of two Boolean operands is true
if at least one of the operands is true. Table 3.7 defines the exclusive or (^) operator. The
exclusive or (^) of two Boolean operands is true if and only if the two operands have different Boolean values. Note that p1 ^ p2 is the same as p1 != p2.
TABLE 3.3 Boolean Operators
Operator
Name
Description
!
not
logical negation
&&
and
logical conjunction
||
or
logical disjunction
^
exclusive or
logical exclusion
TABLE 3.4 Truth Table for Operator !
p
!p
Example (assume age = 24, weight = 140)
true
false
!(age > 18) is false, because (age > 18) is true.
false
true
!(weight == 150) is true, because (weight == 150)
is false.
Key
Point
94 Chapter 3
Selections
TABLE 3.5 Truth Table for Operator &&
p1
p2
p1 && p2
false
false
false
false
true
false
true
false
false
true
true
true
Example (assume age = 24, weight = 140)
(age > 28) && (weight <= 140) is true,
because (age > 28) is false.
(age > 18) && (weight >= 140) is true,
because (age > 18) and (weight >= 140) are
both true.
TABLE 3.6 Truth Table for Operator ||
p1
p2
p1 || p2
Example (assume age = 24, weight = 140)
false
false
false
(age > 34) || (weight >= 150) is false, because
(age > 34) and (weight >= 150) are both false.
false
true
true
true
false
true
true
true
true
(age > 18) || (weight < 140) is true, because
(age > 18) is true.
TABLE 3.7 Truth Table for Operator ^
p1
p2
p1 ^ p2
Example (assume age = 24, weight = 140)
false
false
false
(age > 34) ^ (weight > 140) is false, because (age > 34) and
(weight > 140) are both false.
false
true
true
(age > 34) ^ (weight >= 140) is true, because (age > 34) is
false but (weight >= 140) is true.
true
false
true
true
true
false
Listing 3.6 gives a program that checks whether a number is divisible by 2 and 3, by 2 or
3, and by 2 or 3 but not both:
LISTING 3.6 TestBooleanOperators.java
import class
input
and
1
2
3
4
5
6
7
8
9
10
11
12
13
14
import java.util.Scanner;
public class TestBooleanOperators {
public static void main(String[] args) {
// Create a Scanner
Scanner input = new Scanner(System.in);
// Receive an input
System.out.print("Enter an integer: ");
int number = input.nextInt();
if (number % 2 == 0 && number % 3 == 0)
System.out.println(number + " is divisible by 2 and 3.");
3.10 Logical Operators 95
15
16
17
18
19
20
21
22
if (number % 2 == 0 || number % 3 == 0)
System.out.println(number + " is divisible by 2 or 3.");
or
if (number % 2 == 0 ^ number % 3 == 0)
System.out.println(number +
" is divisible by 2 or 3, but not both.");
exclusive or
}
}
Enter an integer: 4
4 is divisible by 2 or 3.
4 is divisible by 2 or 3, but not both.
Enter an integer: 18
18 is divisible by 2 and 3.
18 is divisible by 2 or 3.
(number % 2 == 0 && number % 3 == 0) (line 12) checks whether the number is
divisible by both 2 and 3. (number % 2 == 0 || number % 3 == 0) (line 15) checks
whether the number is divisible by 2 or by 3. (number % 2 == 0 ^ number % 3 == 0) (line
18) checks whether the number is divisible by 2 or 3, but not both.
Caution
In mathematics, the expression
1 <= numberOfDaysInAMonth <= 31
is correct. However, it is incorrect in Java, because 1 <= numberOfDaysInAMonth is
evaluated to a boolean value, which cannot be compared with 31. Here, two operands
(a boolean value and a numeric value) are incompatible. The correct expression in
Java is
incompatible operands
(1 <= numberOfDaysInAMonth) && (numberOfDaysInAMonth <= 31)
Note
De Morgan’s law, named after Indian-born British mathematician and logician Augustus
De Morgan (1806–1871), can be used to simplify Boolean expressions. The law states:
!(condition1 && condition2) is the same as
!condition1 || !condition2
!(condition1 || condition2) is the same as
!condition1 && !condition2
For example,
! (number % 2 == 0 && number % 3 == 0)
can be simplified using an equivalent expression:
(number % 2 != 0 || number % 3 != 0)
As another example,
!(number == 2 || number == 3)
is better written as
number != 2 && number != 3
De Morgan’s law
96 Chapter 3
Selections
If one of the operands of an && operator is false, the expression is false; if one of the
operands of an || operator is true, the expression is true. Java uses these properties to
improve the performance of these operators. When evaluating p1 && p2, Java first evaluates
p1 and then, if p1 is true, evaluates p2; if p1 is false, it does not evaluate p2. When
evaluating p1 || p2, Java first evaluates p1 and then, if p1 is false, evaluates p2; if p1 is
true, it does not evaluate p2. In programming language terminology, && and || are known
as the short-circuit or lazy operators. Java also provides the unconditional AND (&) and OR
(|) operators, which are covered in Supplement III.C for advanced readers.
short-circuit operator
lazy operator
✓
Check
Point
3.18
Assuming that x is 1, show the result of the following Boolean expressions.
(true) && (3 > 4)
!(x > 0) && (x > 0)
(x > 0) || (x < 0)
(x != 0) || (x == 0)
(x >= 0) || (x < 0)
(x != 1) == !(x == 1)
3.19
(a) Write a Boolean expression that evaluates to true if a number stored in variable
num is between 1 and 100. (b) Write a Boolean expression that evaluates to true if
a number stored in variable num is between 1 and 100 or the number is negative.
3.20
(a) Write a Boolean expression for 0 x - 5 0 6 4.5. (b) Write a Boolean expression
for 0 x - 5 0 7 4.5.
3.21
Assume that x and y are int type. Which of the following are legal Java expressions?
x > y > 0
x = y && y
x /= y
x or y
x and y
(x != 0) || (x = 0)
3.22
Are the following two expressions the same?
a. x % 2 == 0 && x % 3 == 0
b. x % 6 == 0
3.23
3.24
What is the value of the expression x >= 50 && x <= 100 if x is 45, 67, or 101?
Suppose, when you run the following program, you enter the input 2 3 6 from the
console. What is the output?
public class Test {
public static void main(String[] args) {
java.util.Scanner input = new java.util.Scanner(System.in);
double x = input.nextDouble();
double y = input.nextDouble();
double z = input.nextDouble();
System.out.println("(x < y && y
System.out.println("(x < y || y
System.out.println("!(x < y) is
System.out.println("(x + y < z)
System.out.println("(x + y > z)
< z) is "
< z) is "
" + !(x <
is " + (x
is " + (x
+ (x < y && y < z));
+ (x < y || y < z));
y));
+ y < z));
+ y > z));
}
}
3.25
Write a Boolean expression that evaluates to true if age is greater than 13 and less
than 18.
3.11 Case Study: Determining Leap Year 97
3.26
Write a Boolean expression that evaluates to true if weight is greater than
50 pounds or height is greater than 60 inches.
3.27
Write a Boolean expression that evaluates to true if weight is greater than
50 pounds and height is greater than 60 inches.
Write a Boolean expression that evaluates to true if either weight is greater than
50 pounds or height is greater than 60 inches, but not both.
3.28
3.11 Case Study: Determining Leap Year
A year is a leap year if it is divisible by 4 but not by 100, or if it is divisible by 400.
You can use the following Boolean expressions to check whether a year is a leap year:
Key
Point
// A leap year is divisible by 4
boolean isLeapYear = (year % 4 == 0);
// A leap year is divisible by 4 but not by 100
isLeapYear = isLeapYear && (year % 100 != 0);
// A leap year is divisible by 4 but not by 100 or divisible by 400
isLeapYear = isLeapYear || (year % 400 == 0);
Or you can combine all these expressions into one like this:
isLeapYear = (year % 4 == 0 && year % 100 != 0) || (year % 400 == 0);
Listing 3.7 gives the program that lets the user enter a year and checks whether it is a leap
year.
LISTING 3.7 LeapYear.java
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
import java.util.Scanner;
public class LeapYear {
public static void main(String[] args) {
// Create a Scanner
Scanner input = new Scanner(System.in);
System.out.print("Enter a year: ");
int year = input.nextInt();
// Check if the year is a leap year
boolean isLeapYear =
(year % 4 == 0 && year % 100 != 0) || (year % 400 == 0);
// Display the result
System.out.println(year + " is a leap year? " + isLeapYear);
}
}
Enter a year: 2008
2008 is a leap year? true
Enter a year: 1900
1900 is a leap year? false
input
leap year?
display result
98 Chapter 3
Selections
Enter a year: 2002
2002 is a leap year? false
3.12 Case Study: Lottery
Key
Point
The lottery program involves generating random numbers, comparing digits, and
using Boolean operators.
Suppose you want to develop a program to play lottery. The program randomly generates a
lottery of a two-digit number, prompts the user to enter a two-digit number, and determines
whether the user wins according to the following rules:
1. If the user input matches the lottery number in the exact order, the award is $10,000.
2. If all digits in the user input match all digits in the lottery number, the award is $3,000.
3. If one digit in the user input matches a digit in the lottery number, the award is $1,000.
Note that the digits of a two-digit number may be 0. If a number is less than 10, we assume
the number is preceded by a 0 to form a two-digit number. For example, number 8 is treated
as 08 and number 0 is treated as 00 in the program. Listing 3.8 gives the complete program.
LISTING 3.8 Lottery.java
generate a lottery number
enter a guess
exact match?
match all digits?
match one digit?
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
import java.util.Scanner;
public class Lottery {
public static void main(String[] args) {
// Generate a lottery number
int lottery = (int)(Math.random() * 100);
// Prompt the user to enter a guess
Scanner input = new Scanner(System.in);
System.out.print("Enter your lottery pick (two digits): ");
int guess = input.nextInt();
// Get digits from lottery
int lotteryDigit1 = lottery / 10;
int lotteryDigit2 = lottery % 10;
// Get digits from guess
int guessDigit1 = guess / 10;
int guessDigit2 = guess % 10;
System.out.println("The lottery number is " + lottery);
// Check the guess
if (guess == lottery)
System.out.println("Exact match: you win $10,000");
else if (guessDigit2 == lotteryDigit1
&& guessDigit1 == lotteryDigit2)
System.out.println("Match all digits: you win $3,000");
else if (guessDigit1 == lotteryDigit1
|| guessDigit1 == lotteryDigit2
|| guessDigit2 == lotteryDigit1
|| guessDigit2 == lotteryDigit2)
System.out.println("Match one digit: you win $1,000");
3.12 Case Study: Lottery 99
34
35
36
37
else
System.out.println("Sorry, no match");
}
}
Enter your lottery pick (two digits): 15
The lottery number is 15
Exact match: you win $10,000
Enter your lottery pick (two digits): 45
The lottery number is 54
Match all digits: you win $3,000
Enter your lottery pick: 23
The lottery number is 34
Match one digit: you win $1,000
Enter your lottery pick: 23
The lottery number is 14
Sorry: no match
line#
6
11
14
15
18
19
33
variable
lottery
guess
lotteryDigit1
lotteryDigit2
guessDigit1
guessDigit2
34
23
3
4
2
3
Output
Match one digit:
you win $1,000
The program generates a lottery using the random() method (line 6) and prompts the user
to enter a guess (line 11). Note that guess % 10 obtains the last digit from guess and guess
/ 10 obtains the first digit from guess, since guess is a two-digit number (lines 18–19).
The program checks the guess against the lottery number in this order:
1. First, check whether the guess matches the lottery exactly (line 24).
2. If not, check whether the reversal of the guess matches the lottery (lines 26–27).
3. If not, check whether one digit is in the lottery (lines 29–32).
4. If not, nothing matches and display "Sorry, no match" (lines 34–35).
100 Chapter 3
Selections
3.13 switch Statements
Key
Point
A switch statement executes statements based on the value of a variable or an
expression.
The if statement in Listing 3.5, ComputeTax.java, makes selections based on a single true
or false condition. There are four cases for computing taxes, which depend on the value of
status. To fully account for all the cases, nested if statements were used. Overuse of nested
if statements makes a program difficult to read. Java provides a switch statement to simplify coding for multiple conditions. You can write the following switch statement to replace
the nested if statement in Listing 3.5:
switch (status) {
case 0: compute tax for single filers;
break;
case 1: compute tax for married jointly or qualifying widow(er);
break;
case 2: compute tax for married filing separately;
break;
case 3: compute tax for head of household;
break;
default: System.out.println("Error: invalid status");
System.exit(1);
}
The flowchart of the preceding switch statement is shown in Figure 3.5.
status is 0
status is 1
status is 2
status is 3
default
Compute tax for single filers
Compute tax for married jointly or qualifying widow(er)
break
break
Compute tax for married filing separately
break
Compute tax for head of household
break
Default actions
FIGURE 3.5 The switch statement checks all cases and executes the statements in the
matched case.
switch statement
This statement checks to see whether the status matches the value 0, 1, 2, or 3, in that
order. If matched, the corresponding tax is computed; if not matched, a message is displayed.
Here is the full syntax for the switch statement:
switch (switch-expression) {
case value1: statement(s)1;
break;
3.13 switch Statements 101
case value2: statement(s)2;
break;
...
case valueN: statement(s)N;
break;
default:
statement(s)-for-default;
}
The switch statement observes the following rules:
■
The switch-expression must yield a value of char, byte, short, int, or String
type and must always be enclosed in parentheses. (The char and String types will be
introduced in the next chapter.)
■
The value1, . . ., and valueN must have the same data type as the value of the switchexpression. Note that value1, . . ., and valueN are constant expressions, meaning
that they cannot contain variables, such as 1 + x.
■
When the value in a case statement matches the value of the switch-expression,
the statements starting from this case are executed until either a break statement or the
end of the switch statement is reached.
■
The default case, which is optional, can be used to perform actions when none of the
specified cases matches the switch-expression.
■
The keyword break is optional. The break statement immediately ends the switch
statement.
Caution
Do not forget to use a break statement when one is needed. Once a case is matched,
the statements starting from the matched case are executed until a break statement or
the end of the switch statement is reached. This is referred to as fall-through behavior.
For example, the following code displays Weekdays for day of 1 to 5 and Weekends
for day 0 and 6.
switch
case
case
case
case
case
case
case
}
(day) {
1:
2:
3:
4:
5: System.out.println("Weekday"); break;
0:
6: System.out.println("Weekend");
Tip
To avoid programming errors and improve code maintainability, it is a good idea to put
a comment in a case clause if break is purposely omitted.
Now let us write a program to find out the Chinese Zodiac sign for a given year. The
Chinese Zodiac is based on a twelve-year cycle, with each year represented by an animal—
monkey, rooster, dog, pig, rat, ox, tiger, rabbit, dragon, snake, horse, or sheep—in this cycle,
as shown in Figure 3.6.
Note that year % 12 determines the Zodiac sign. 1900 is the year of the rat because 1900
% 12 is 4. Listing 3.9 gives a program that prompts the user to enter a year and displays the
animal for the year.
without break
fall-through behavior
102 Chapter 3
Selections
pig
rat
ox
dog
tiger
rooster
year % 12 =
rabbit
monkey
dragon
sheep
horse
FIGURE 3.6
snake
0: monkey
1: rooster
2: dog
3: pig
4: rat
5: ox
6: tiger
7: rabbit
8: dragon
9: snake
10: horse
11: sheep
The Chinese Zodiac is based on a twelve-year cycle.
LISTING 3.9 ChineseZodiac.java
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
enter year
determine Zodiac sign
✓
Check
Point
import java.util.Scanner;
public class ChineseZodiac {
public static void main(String[] args) {
Scanner input = new Scanner(System.in);
System.out.print("Enter a year: ");
int year = input.nextInt();
switch
case
case
case
case
case
case
case
case
case
case
case
case
}
(year % 12) {
0: System.out.println("monkey"); break;
1: System.out.println("rooster"); break;
2: System.out.println("dog"); break;
3: System.out.println("pig"); break;
4: System.out.println("rat"); break;
5: System.out.println("ox"); break;
6: System.out.println("tiger"); break;
7: System.out.println("rabbit"); break;
8: System.out.println("dragon"); break;
9: System.out.println("snake"); break;
10: System.out.println("horse"); break;
11: System.out.println("sheep");
}
}
Enter a year:
rabbit
1963
Enter a year:
ox
1877
3.29
What data types are required for a switch variable? If the keyword break is not
used after a case is processed, what is the next statement to be executed? Can you
convert a switch statement to an equivalent if statement, or vice versa? What are
the advantages of using a switch statement?
3.14 Conditional Expressions 103
3.30
What is y after the following switch statement is executed? Rewrite the code using
an if-else statement.
x = 3; y = 3;
switch (x + 3) {
case 6: y = 1;
default: y += 1;
}
3.31
What is x after the following if-else statement is executed? Use a switch statement to rewrite it and draw the flowchart for the new switch statement.
int x = 1, a = 3;
if (a == 1)
x += 5;
else if (a == 2)
x += 10;
else if (a == 3)
x += 16;
else if (a == 4)
x += 34;
3.32
Write a switch statement that displays Sunday, Monday, Tuesday, Wednesday,
Thursday, Friday, Saturday, if day is 0, 1, 2, 3, 4, 5, 6, accordingly.
3.14 Conditional Expressions
A conditional expression evaluates an expression based on a condition.
You might want to assign a value to a variable that is restricted by certain conditions. For
example, the following statement assigns 1 to y if x is greater than 0, and -1 to y if x is less
than or equal to 0.
Key
Point
if (x > 0)
y = 1;
else
y = -1;
Alternatively, as in the following example, you can use a conditional expression to achieve
the same result.
y = (x > 0) ? 1 : -1;
Conditional expressions are in a completely different style, with no explicit if in the statement. The syntax is:
boolean-expression ? expression1 : expression2;
The result of this conditional expression is expression1 if boolean-expression is true;
otherwise the result is expression2.
Suppose you want to assign the larger number of variable num1 and num2 to max. You can
simply write a statement using the conditional expression:
max = (num1 > num2) ? num1 : num2;
For another example, the following statement displays the message “num is even” if num is
even, and otherwise displays “num is odd.”
System.out.println((num % 2 == 0) ? "num is even" : "num is odd");
conditional expression
104 Chapter 3
Selections
As you can see from these examples, conditional expressions enable you to write short and
concise code.
Note
The symbols ? and : appear together in a conditional expression. They form a
conditional operator and also called a ternary operator because it uses three operands.
It is the only ternary operator in Java.
conditional operator
ternary operator
✓
Check
Point
3.33
Suppose that, when you run the following program, you enter the input 2 3 6 from
the console. What is the output?
public class Test {
public static void main(String[] args) {
java.util.Scanner input = new java.util.Scanner(System.in);
double x = input.nextDouble();
double y = input.nextDouble();
double z = input.nextDouble();
System.out.println((x < y && y < z) ? "sorted" : "not sorted");
}
}
3.34
Rewrite the following if statements using the conditional operator.
if (ages >= 16)
ticketPrice = 20;
else
ticketPrice = 10;
3.35
Rewrite the following conditional expressions using if-else statements.
a. score = (x > 10) ? 3 * scale : 4 * scale;
b. tax = (income > 10000) ? income * 0.2 : income * 0.17 + 1000;
c. System.out.println((number % 3 == 0) ? i : j);
3.36
Write conditional expression that returns -1 or 1 randomly.
3.15 Operator Precedence and Associativity
Key
Point
Operator precedence and associativity determine the order in which operators are
evaluated.
Section 2.11 introduced operator precedence involving arithmetic operators. This section
discusses operator precedence in more detail. Suppose that you have this expression:
3 + 4 * 4 > 5 * (4 + 3) – 1 && (4 - 3 > 5)
operator precedence
What is its value? What is the execution order of the operators?
The expression within parentheses is evaluated first. (Parentheses can be nested, in which
case the expression within the inner parentheses is executed first.) When evaluating an expression without parentheses, the operators are applied according to the precedence rule and the
associativity rule.
The precedence rule defines precedence for operators, as shown in Table 3.8, which contains the operators you have learned so far. Operators are listed in decreasing order of precedence from top to bottom. The logical operators have lower precedence than the relational
operators and the relational operators have lower precedence than the arithmetic operators.
Operators with the same precedence appear in the same group. (See Appendix C, Operator
Precedence Chart, for a complete list of Java operators and their precedence.)
3.15 Operator Precedence and Associativity 105
TABLE 3.8 Operator Precedence Chart
Precedence
Operator
var++ and var–– (Postfix)
+, – (Unary plus and minus), ++var and ––var (Prefix)
(type) (Casting)
!(Not)
*, /, % (Multiplication, division, and remainder)
+, – (Binary addition and subtraction)
<, <=, >, >= (Relational)
==, != (Equality)
^ (Exclusive OR)
&& (AND)
|| (OR)
=, +=, –=, *=, /=, %= (Assignment operator)
If operators with the same precedence are next to each other, their associativity determines
the order of evaluation. All binary operators except assignment operators are left associative.
For example, since + and – are of the same precedence and are left associative, the expression
operator associativity
is equivalent to
a - b + c – d
((a - b) + c) - d
Assignment operators are right associative. Therefore, the expression
is equivalent to
a = b += c = 5
a = (b += (c = 5))
Suppose a, b, and c are 1 before the assignment; after the whole expression is evaluated, a
becomes 6, b becomes 6, and c becomes 5. Note that left associativity for the assignment
operator would not make sense.
Note
Java has its own way to evaluate an expression internally. The result of a Java evaluation
is the same as that of its corresponding arithmetic evaluation. Advanced readers may
refer to Supplement III.B for more discussions on how an expression is evaluated in Java
behind the scenes.
3.37
List the precedence order of the Boolean operators. Evaluate the following expressions:
true || true && false
true && true || false
3.38
3.39
True or false? All the binary operators except = are left associative.
Evaluate the following expressions:
2 * 2 - 3 > 2 && 4 – 2 > 5
2 * 2 - 3 > 2 || 4 – 2 > 5
behind the scenes
✓
Check
Point
106 Chapter 3
Selections
3.40
Is (x > 0 && x < 10) the same as ((x > 0) && (x < 10))? Is (x > 0 ||
x < 10) the same as ((x > 0) || (x < 10))? Is (x > 0 || x < 10 && y
< 0) the same as (x > 0 || (x < 10 && y < 0))?
3.16 Debugging
Key
Point
bugs
debugging
hand-traces
Debugging is the process of finding and fixing errors in a program.
As mentioned in Section 1.10.1, syntax errors are easy to find and easy to correct because the
compiler gives indications as to where the errors came from and why they are there. Runtime
errors are not difficult to find either, because the Java interpreter displays them on the console
when the program aborts. Finding logic errors, on the other hand, can be very challenging.
Logic errors are called bugs. The process of finding and correcting errors is called
debugging. A common approach to debugging is to use a combination of methods to help
pinpoint the part of the program where the bug is located. You can hand-trace the program
(i.e., catch errors by reading the program), or you can insert print statements in order to show
the values of the variables or the execution flow of the program. These approaches might work
for debugging a short, simple program, but for a large, complex program, the most effective
approach is to use a debugger utility.
JDK includes a command-line debugger, jdb, which is invoked with a class name. jdb is
itself a Java program, running its own copy of Java interpreter. All the Java IDE tools, such
as Eclipse and NetBeans, include integrated debuggers. The debugger utilities let you follow
the execution of a program. They vary from one system to another, but they all support most
of the following helpful features.
■
Executing a single statement at a time: The debugger allows you to execute one
statement at a time so that you can see the effect of each statement.
■
Tracing into or stepping over a method: If a method is being executed, you
can ask the debugger to enter the method and execute one statement at a time in the
method, or you can ask it to step over the entire method. You should step over the
entire method if you know that the method works. For example, always step over
system-supplied methods, such as System.out.println.
■
Setting breakpoints: You can also set a breakpoint at a specific statement. Your
program pauses when it reaches a breakpoint. You can set as many breakpoints as
you want. Breakpoints are particularly useful when you know where your programming error starts. You can set a breakpoint at that statement and have the program
execute until it reaches the breakpoint.
■
Displaying variables: The debugger lets you select several variables and display
their values. As you trace through a program, the content of a variable is continuously
updated.
■
Displaying call stacks: The debugger lets you trace all of the method calls. This
feature is helpful when you need to see a large picture of the program-execution flow.
■
Modifying variables: Some debuggers enable you to modify the value of a variable when debugging. This is convenient when you want to test a program with different samples but do not want to leave the debugger.
Tip
debugging in IDE
If you use an IDE such as Eclipse or NetBeans, please refer to Learning Java Effectively
with Eclipse/NetBeans in Supplements II.C and II.E on the Companion Website. The
supplement shows you how to use a debugger to trace programs and how debugging
can help in learning Java effectively.
Chapter Summary 107
KEY TERMS
Boolean expression 76
boolean data type 76
Boolean value 76
conditional operator 104
dangling else ambiguity 85
debugging 106
fall-through behavior 101
flowchart 78
lazy operator 96
operator associativity 105
operator precedence 104
selection statement 76
short-circuit operator 96
CHAPTER SUMMARY
1. A boolean type variable can store a true or false value.
2. The relational operators (<, <=, ==, !=, >, >=) yield a Boolean value.
3. Selection statements are used for programming with alternative courses of actions.
There are several types of selection statements: one-way if statements, two-way
if-else statements, nested if statements, multi-way if-else statements, switch
statements, and conditional expressions.
4. The various if statements all make control decisions based on a Boolean expression.
Based on the true or false evaluation of the expression, these statements take one of
two possible courses.
5. The Boolean operators &&, ||, !, and ^ operate with Boolean values and variables.
6. When evaluating p1
&& p2, Java first evaluates p1 and then evaluates p2 if p1 is
true; if p1 is false, it does not evaluate p2. When evaluating p1 || p2, Java first
evaluates p1 and then evaluates p2 if p1 is false; if p1 is true, it does not evaluate
p2. Therefore, && is referred to as the conditional or short-circuit AND operator, and
|| is referred to as the conditional or short-circuit OR operator.
7. The switch statement makes control decisions based on a switch expression of type
char, byte, short, int, or String.
8. The keyword break is optional in a switch statement, but it is normally used at the
end of each case in order to skip the remainder of the switch statement. If the break
statement is not present, the next case statement will be executed.
9. The operators in expressions are evaluated in the order determined by the rules of
parentheses, operator precedence, and operator associativity.
10. Parentheses can be used to force the order of evaluation to occur in any sequence.
11. Operators with higher precedence are evaluated earlier. For operators of the same
precedence, their associativity determines the order of evaluation.
12. All binary operators except assignment operators are left-associative; assignment
operators are right-associative.
108 Chapter 3
Selections
TEST QUESTIONS
Answer the quiz for this chapter online at www.cs.armstrong.edu/liang/intro10e/quiz.html.
PROGRAMMING EXERCISES
Pedagogical Note
think before coding
For each exercise, carefully analyze the problem requirements and design strategies for
solving the problem before coding.
Debugging Tip
Before you ask for help, read and explain the program to yourself, and trace it using
several representative inputs by hand or using an IDE debugger. You learn how to
program by debugging your own mistakes.
learn from mistakes
Section 3.2
*3.1
(Algebra: solve quadratic equations) The two roots of a quadratic equation
ax2 + bx + c = 0 can be obtained using the following formula:
r1 =
-b + 2b2 - 4ac
2a
and r2 =
-b - 2b2 - 4ac
2a
b2 - 4ac is called the discriminant of the quadratic equation. If it is positive, the
equation has two real roots. If it is zero, the equation has one root. If it is negative,
the equation has no real roots.
Write a program that prompts the user to enter values for a, b, and c and displays
the result based on the discriminant. If the discriminant is positive, display two
roots. If the discriminant is 0, display one root. Otherwise, display “The equation
has no real roots”.
Note that you can use Math.pow(x, 0.5) to compute 2x. Here are some
sample runs.
Enter a, b, c: 1.0 3 1
The equation has two roots -0.381966 and -2.61803
Enter a, b, c: 1 2.0 1
The equation has one root -1
Enter a, b, c: 1 2 3
The equation has no real roots
3.2
(Game: add three numbers) The program in Listing 3.1, AdditionQuiz.java, generates two integers and prompts the user to enter the sum of these two integers.
Revise the program to generate three single-digit integers and prompt the user to
enter the sum of these three integers.
Programming Exercises 109
Sections 3.3–3.7
*3.3
(Algebra: solve 2 * 2 linear equations) A linear equation can be solved using
Cramer’s rule given in Programming Exercise 1.13. Write a program that prompts
the user to enter a, b, c, d, e, and f and displays the result. If ad - bc is 0, report
that “The equation has no solution.”
Enter a, b, c, d, e, f: 9.0 4.0 3.0 -5.0 -6.0 -21.0
x is -2.0 and y is 3.0
Enter a, b, c, d, e, f: 1.0 2.0 2.0 4.0 4.0 5.0
The equation has no solution
**3.4
*3.5
(Random month) Write a program that randomly generates an integer between 1
and 12 and displays the English month name January, February, …, December for
the number 1, 2, …, 12, accordingly.
(Find future dates) Write a program that prompts the user to enter an integer for
today’s day of the week (Sunday is 0, Monday is 1, …, and Saturday is 6). Also
prompt the user to enter the number of days after today for a future day and display the future day of the week. Here is a sample run:
Enter today's day: 1
Enter the number of days elapsed since today: 3
Today is Monday and the future day is Thursday
Enter today's day: 0
Enter the number of days elapsed since today: 31
Today is Sunday and the future day is Wednesday
*3.6
(Health application: BMI) Revise Listing 3.4, ComputeAndInterpretBMI.java, to
let the user enter weight, feet, and inches. For example, if a person is 5 feet and 10
inches, you will enter 5 for feet and 10 for inches. Here is a sample run:
Enter weight in pounds: 140
Enter feet: 5
Enter inches: 10
BMI is 20.087702275404553
Normal
3.7
(Financial application: monetary units) Modify Listing 2.10, ComputeChange
.java, to display the nonzero denominations only, using singular words for single
units such as 1 dollar and 1 penny, and plural words for more than one unit such
as 2 dollars and 3 pennies.
110 Chapter 3
VideoNote
Sort three integers
Selections
*3.8
(Sort three integers) Write a program that prompts the user to enter three integers
and display the integers in non-decreasing order.
**3.9
(Business: check ISBN-10) An ISBN-10 (International Standard Book Number)
consists of 10 digits: d1d2d3d4d5d6d7d8d9d10. The last digit, d10, is a checksum,
which is calculated from the other nine digits using the following formula:
(d1 * 1 + d2 * 2 + d3 * 3 + d4 * 4 + d5 * 5 +
d6 * 6 + d7 * 7 + d8 * 8 + d9 * 9) % 11
If the checksum is 10, the last digit is denoted as X according to the ISBN-10
convention. Write a program that prompts the user to enter the first 9 digits and
displays the 10-digit ISBN (including leading zeros). Your program should read
the input as an integer. Here are sample runs:
Enter the first 9 digits of an ISBN as integer: 013601267
The ISBN-10 number is 0136012671
Enter the first 9 digits of an ISBN as integer: 013031997
The ISBN-10 number is 013031997X
3.10
(Game: addition quiz) Listing 3.3, SubtractionQuiz.java, randomly generates a
subtraction question. Revise the program to randomly generate an addition question with two integers less than 100.
Sections 3.8–3.16
*3.11
(Find the number of days in a month) Write a program that prompts the user
to enter the month and year and displays the number of days in the month. For
example, if the user entered month 2 and year 2012, the program should display
that February 2012 had 29 days. If the user entered month 3 and year 2015, the
program should display that March 2015 had 31 days.
3.12
(Palindrome number) Write a program that prompts the user to enter a three-digit
integer and determines whether it is a palindrome number. A number is palindrome if it reads the same from right to left and from left to right. Here is a sample
run of this program:
Enter a three-digit integer: 121
121 is a palindrome
Enter a three-digit integer: 123
123 is not a palindrome
*3.13
3.14
(Financial application: compute taxes) Listing 3.5, ComputeTax.java, gives the
source code to compute taxes for single filers. Complete Listing 3.5 to compute
the taxes for all filing statuses.
(Game: heads or tails) Write a program that lets the user guess whether the flip of
a coin results in heads or tails. The program randomly generates an integer 0 or 1,
which represents head or tail. The program prompts the user to enter a guess and
reports whether the guess is correct or incorrect.
Programming Exercises 111
**3.15
3.16
*3.17
(Game: lottery) Revise Listing 3.8, Lottery.java, to generate a lottery of a threedigit number. The program prompts the user to enter a three-digit number and
determines whether the user wins according to the following rules:
1. If the user input matches the lottery number in the exact order, the award is
$10,000.
2. If all digits in the user input match all digits in the lottery number, the award is
$3,000.
3. If one digit in the user input matches a digit in the lottery number, the award is
$1,000.
(Random point) Write a program that displays a random coordinate in a rectangle.
The rectangle is centered at (0, 0) with width 100 and height 200.
(Game: scissor, rock, paper) Write a program that plays the popular scissor-rockpaper game. (A scissor can cut a paper, a rock can knock a scissor, and a paper can
wrap a rock.) The program randomly generates a number 0, 1, or 2 representing
scissor, rock, and paper. The program prompts the user to enter a number 0, 1, or
2 and displays a message indicating whether the user or the computer wins, loses,
or draws. Here are sample runs:
scissor (0), rock (1), paper (2): 1
The computer is scissor. You are rock. You won
scissor (0), rock (1), paper (2): 2
The computer is paper. You are paper too. It is a draw
*3.18
(Cost of shipping) A shipping company uses the following function to calculate the cost (in dollars) of shipping based on the weight of the package (in
pounds).
3.5, if 0 6
5.5, if 1 6
c(w) = d
8.5, if 3 6
10.5, if 10
**3.19
*3.20
w
w
w
6
6= 1
6= 3
6 = 10
w 6 = 20
Write a program that prompts the user to enter the weight of the package and
display the shipping cost. If the weight is greater than 50, display a message “the
package cannot be shipped.”
(Compute the perimeter of a triangle) Write a program that reads three edges for
a triangle and computes the perimeter if the input is valid. Otherwise, display that
the input is invalid. The input is valid if the sum of every pair of two edges is
greater than the remaining edge.
(Science: wind-chill temperature) Programming Exercise 2.17 gives a formula
to compute the wind-chill temperature. The formula is valid for temperatures in
the range between −58ºF and 41ºF and wind speed greater than or equal to 2.
Write a program that prompts the user to enter a temperature and a wind speed.
The program displays the wind-chill temperature if the input is valid; otherwise,
it displays a message indicating whether the temperature and/or wind speed is
invalid.
112 Chapter 3
Selections
Comprehensive
**3.21
(Science: day of the week) Zeller’s congruence is an algorithm developed by
Christian Zeller to calculate the day of the week. The formula is
h = ¢q +
j
26(m + 1)
k
+ k + + + 5j≤ % 7
10
4
4
where
■ h
is the day of the week (0: Saturday, 1: Sunday, 2: Monday, 3: Tuesday, 4:
Wednesday, 5: Thursday, 6: Friday).
■ q
is the day of the month.
■ m
is the month (3: March, 4: April, …, 12: December). January and February
are counted as months 13 and 14 of the previous year.
year
■ j is the century (i.e.,
).
100
■ k is the year of the century (i.e., year % 100).
Note that the division in the formula performs an integer division. Write a program that prompts the user to enter a year, month, and day of the month, and
displays the name of the day of the week. Here are some sample runs:
Enter year: (e.g., 2012): 2015
Enter month: 1-12: 1
Enter the day of the month: 1-31: 25
Day of the week is Sunday
Enter year: (e.g., 2012): 2012
Enter month: 1-12: 5
Enter the day of the month: 1-31: 12
Day of the week is Saturday
**3.22
VideoNote
Check point location
(Hint: January and February are counted as 13 and 14 in the formula, so you need
to convert the user input 1 to 13 and 2 to 14 for the month and change the year to
the previous year.)
(Geometry: point in a circle?) Write a program that prompts the user to enter a
point (x, y) and checks whether the point is within the circle centered at (0, 0)
with radius 10. For example, (4, 5) is inside the circle and (9, 9) is outside the
circle, as shown in Figure 3.7a.
(Hint: A point is in the circle if its distance to (0, 0) is less than or equal to 10.
The formula for computing the distance is 2(x2 - x1)2 + (y2 - y1)2. Test your
program to cover all cases.) Two sample runs are shown below.
Enter a point with two coordinates: 4 5
Point (4.0, 5.0) is in the circle
Enter a point with two coordinates: 9 9
Point (9.0, 9.0) is not in the circle
Programming Exercises 113
y-axis
y-axis
(9, 9)
(4, 5)
(6, 4)
(2, 2)
(0, 0)
x-axis
(a)
(0, 0)
x-axis
(b)
FIGURE 3.7 (a) Points inside and outside of the circle. (b) Points inside and outside of the
rectangle.
**3.23
(Geometry: point in a rectangle?) Write a program that prompts the user to enter
a point (x, y) and checks whether the point is within the rectangle centered at
(0, 0) with width 10 and height 5. For example, (2, 2) is inside the rectangle and
(6, 4) is outside the rectangle, as shown in Figure 3.7b. (Hint: A point is in the
rectangle if its horizontal distance to (0, 0) is less than or equal to 10 / 2 and its
vertical distance to (0, 0) is less than or equal to 5.0 / 2. Test your program to
cover all cases.) Here are two sample runs.
Enter a point with two coordinates: 2 2
Point (2.0, 2.0) is in the rectangle
Enter a point with two coordinates: 6 4
Point (6.0, 4.0) is not in the rectangle
**3.24
(Game: pick a card) Write a program that simulates picking a card from a deck
of 52 cards. Your program should display the rank (Ace, 2, 3, 4, 5, 6, 7, 8, 9, 10,
Jack, Queen, King) and suit (Clubs, Diamonds, Hearts, Spades) of the card.
Here is a sample run of the program:
The card you picked is Jack of Hearts
*3.25
(Geometry: intersecting point) Two points on line 1 are given as (x1, y1) and (x2,
y2) and on line 2 as (x3, y3) and (x4, y4), as shown in Figure 3.8a–b.
The intersecting point of the two lines can be found by solving the following
linear equation:
(y1 - y2)x - (x1 - x2)y = (y1 - y2)x1 - (x1 - x2)y1
(y3 - y4)x - (x3 - x4)y = (y3 - y4)x3 - (x3 - x4)y3
This linear equation can be solved using Cramer’s rule (see Programming Exercise 3.3). If the equation has no solutions, the two lines are parallel (Figure 3.8c).
114 Chapter 3
Selections
Write a program that prompts the user to enter four points and displays the intersecting point. Here are sample runs:
(x2, y2)
(x2, y2)
(x2, y2) (x3, y3)
(x3, y3)
(x3, y3)
(x4, y4)
(x1, y1)
FIGURE 3.8
(x4, y4)
(x1, y1)
(a)
(x1, y1)
(b)
(x4, y4)
(c)
Two lines intersect in (a and b) and two lines are parallel in (c).
Enter x1, y1, x2, y2, x3, y3, x4, y4: 2 2 5 -1.0 4.0 2.0 -1.0 -2.0
The intersecting point is at (2.88889, 1.1111)
Enter x1, y1, x2, y2, x3, y3, x4, y4: 2 2 7 6.0 4.0 2.0 -1.0 -2.0
The two lines are parallel
3.26
(Use the &&, || and ^ operators) Write a program that prompts the user to enter
an integer and determines whether it is divisible by 5 and 6, whether it is divisible
by 5 or 6, and whether it is divisible by 5 or 6, but not both. Here is a sample run
of this program:
Enter
Is 10
Is 10
Is 10
**3.27
an integer: 10
divisible by 5 and 6? false
divisible by 5 or 6? true
divisible by 5 or 6, but not both? true
(Geometry: points in triangle?) Suppose a right triangle is placed in a plane as
shown below. The right-angle point is placed at (0, 0), and the other two points
are placed at (200, 0), and (0, 100). Write a program that prompts the user to enter
a point with x- and y-coordinates and determines whether the point is inside the
triangle. Here are the sample runs:
(0, 100)
p2
p1
(0, 0)
(200, 0)
Enter a point's x- and y-coordinates: 100.5 25.5
The point is in the triangle
Programming Exercises 115
Enter a point's x- and y-coordinates: 100.5 50.5
The point is not in the triangle
**3.28
(Geometry: two rectangles) Write a program that prompts the user to enter the
center x-, y-coordinates, width, and height of two rectangles and determines
whether the second rectangle is inside the first or overlaps with the first, as shown
in Figure 3.9. Test your program to cover all cases.
w1
w1
w2
h1 h2
(x1, y1)
h1
w2
(x1, y1)
(x2, y2)
h2
(a)
(x2, y2)
(b)
FIGURE 3.9 (a) A rectangle is inside another one. (b) A rectangle overlaps another one.
Here are the sample runs:
Enter r1's center x-, y-coordinates, width, and height: 2.5 4 2.5 43
Enter r2's center x-, y-coordinates, width, and height: 1.5 5 0.5 3
r2 is inside r1
Enter r1's center x-, y-coordinates, width, and height: 1 2 3 5.5
Enter r2's center x-, y-coordinates, width, and height: 3 4 4.5 5
r2 overlaps r1
Enter r1's center x-, y-coordinates, width, and height: 1 2 3 3
Enter r2's center x-, y-coordinates, width, and height: 40 45 3 2
r2 does not overlap r1
**3.29
(Geometry: two circles) Write a program that prompts the user to enter the center
coordinates and radii of two circles and determines whether the second circle is
inside the first or overlaps with the first, as shown in Figure 3.10. (Hint: circle2 is
inside circle1 if the distance between the two centers 6 = |r1 - r2| and circle2
overlaps circle1 if the distance between the two centers <= r1 + r2. Test your
program to cover all cases.)
Here are the sample runs:
Enter circle1's center x-, y-coordinates, and radius: 0.5 5.1 13
Enter circle2's center x-, y-coordinates, and radius: 1 1.7 4.5
circle2 is inside circle1
116 Chapter 3
Selections
r1
r1
(x1, y1)
(x1, y1)
r2
r2
(x2, y2)
(a)
(x2, y2)
(b)
FIGURE 3.10 (a) A circle is inside another circle. (b) A circle overlaps another circle.
Enter circle1's center x-, y-coordinates, and radius: 3.4 5.7 5.5
Enter circle2's center x-, y-coordinates, and radius: 6.7 3.5 3
circle2 overlaps circle1
Enter circle1's center x-, y-coordinates, and radius: 3.4 5.5 1
Enter circle2's center x-, y-coordinates, and radius: 5.5 7.2 1
circle2 does not overlap circle1
*3.30
(Current time) Revise Programming Exercise 2.8 to display the hour using a
12-hour clock. Here is a sample run:
Enter the time zone offset to GMT: -5
The current time is 4:50:34 AM
*3.31
(Financials: currency exchange) Write a program that prompts the user to enter
the exchange rate from currency in U.S. dollars to Chinese RMB. Prompt the user
to enter 0 to convert from U.S. dollars to Chinese RMB and 1 to convert from
Chinese RMB and U.S. dollars. Prompt the user to enter the amount in U.S. dollars or Chinese RMB to convert it to Chinese RMB or U.S. dollars, respectively.
Here are the sample runs:
Enter the exchange rate from dollars to RMB: 6.81
Enter 0 to convert dollars to RMB and 1 vice versa: 0
Enter the dollar amount: 100
$100.0 is 681.0 yuan
Enter the exchange rate from dollars to RMB: 6.81
Enter 0 to convert dollars to RMB and 1 vice versa: 5
Enter the RMB amount: 10000
10000.0 yuan is $1468.43
Programming Exercises 117
Enter the exchange rate from dollars to RMB: 6.81
Enter 0 to convert dollars to RMB and 1 vice versa: 5
Incorrect input
*3.32
(Geometry: point position) Given a directed line from point p0(x0, y0) to p1(x1,
y1), you can use the following condition to decide whether a point p2(x2, y2) is
on the left of the line, on the right, or on the same line (see Figure 3.11):
7 0 p2 is on the left side of the line
(x1 - x0)*(y2 - y0) - (x2 - x0)*(y1 - y0) c =0 p2 is on the same line
6 0 p2 is on the right side of the line
p1
p1
p2
p1
p2
p2
p0
(a)
p0
p0
(b)
(c)
FIGURE 3.11 (a) p2 is on the left of the line. (b) p2 is on the right of the line. (c) p2 is on
the same line.
Write a program that prompts the user to enter the three points for p0, p1, and p2
and displays whether p2 is on the left of the line from p0 to p1, on the right, or on
the same line. Here are some sample runs:
Enter three points for p0, p1, and p2: 4.4 2 6.5 9.5 -5 4
(-5.0, 4.0) is on the left side of the line from (4.4, 2.0) to (6.5, 9.5)
Enter three points for p0, p1, and p2: 1 1 5 5 2 2
(2.0, 2.0) is on the line from (1.0, 1.0) to (5.0, 5.0)
Enter three points for p0, p1, and p2: 3.4 2 6.5 9.5 5 2.5
(5.0, 2.5) is on the right side of the line from (3.4, 2.0) to (6.5, 9.5)
*3.33
(Financial: compare costs) Suppose you shop for rice in two different packages.
You would like to write a program to compare the cost. The program prompts the
user to enter the weight and price of the each package and displays the one with
the better price. Here is a sample run:
Enter weight and price for package 1: 50 24.59
Enter weight and price for package 2: 25 11.99
Package 2 has a better price.
118 Chapter 3
Selections
Enter weight and price for package 1: 50 25
Enter weight and price for package 2: 25 12.5
Two packages have the same price.
*3.34
(Geometry: point on line segment) Programming Exercise 3.32 shows how to test
whether a point is on an unbounded line. Revise Programming Exercise 3.32 to
test whether a point is on a line segment. Write a program that prompts the user
to enter the three points for p0, p1, and p2 and displays whether p2 is on the line
segment from p0 to p1. Here are some sample runs:
Enter three points for p0, p1, and p2: 1 1 2.5 2.5 1.5 1.5
(1.5, 1.5) is on the line segment from (1.0, 1.0) to (2.5, 2.5)
Enter three points for p0, p1, and p2: 1 1 2 2 3.5 3.5
(3.5, 3.5) is not on the line segment from (1.0, 1.0) to (2.0, 2.0)
CHAPTER
MATHEMATICAL
FUNCTIONS,
CHARACTERS,
AND STRINGS
Objectives
■
■
■
■
■
To solve mathematical problems by using the methods in the Math class (§4.2).
To represent characters using the char type (§4.3).
To encode characters using ASCII and Unicode (§4.3.1).
To represent special characters using the escape sequences (§4.4.2).
To cast a numeric value to a character and cast a character to an integer (§4.3.3).
■
To compare and test characters using the static methods in the Character
class (§4.3.4).
■
To introduce objects and instance methods (§4.4).
■
To represent strings using the String object (§4.4).
To return the string length using the length() method (§4.4.1).
To return a character in the string using the charAt(i) method (§4.4.2).
To use the + operator to concatenate strings (§4.4.3).
■
■
■
■
■
■
■
■
■
■
■
■
■
To return an uppercase string or a lowercase string and to trim a string (§4.4.4).
To read strings from the console (§4.4.5).
To read a character from the console (§4.4.6).
To compare strings using the equals method and the compareTo methods
(§4.4.7).
To obtain substrings (§4.4.8).
To find a character or a substring in a string using the indexOf method (§4.4.9).
To program using characters and strings (GuessBirthday) (§4.5.1).
To convert a hexadecimal character to a decimal value (HexDigit2Dec)
(§4.5.2).
To revise the lottery program using strings (LotteryUsingStrings) (§4.5.3).
To format output using the System.out.printf method (§4.6).
4
120 Chapter 4
Mathematical Functions, Characters, and Strings
4.1 Introduction
Key
Point
The focus of this chapter is to introduce mathematical functions, characters, string
objects, and use them to develop programs.
The preceding chapters introduced fundamental programming techniques and taught you how
to write simple programs to solve basic problems using selection statements. This chapter introduces methods for performing common mathematical operations. You will learn how to create
custom methods in Chapter 6.
Suppose you need to estimate the area enclosed by four cities, given the GPS locations (latitude
and longitude) of these cities, as shown in the following diagram. How would you write a program
to solve this problem? You will be able to write such a program after completing this chapter.
problem
Charlotte (35.2270869, –80.8431267)
Atlanta
(33.7489954, –84.3879824)
Savannah (32.0835407, –81.0998342)
Orlando (28.5383355, –81.3792365)
Because strings are frequently used in programming, it is beneficial to introduce strings
early so that you can begin to use them to develop useful programs. This chapter gives a brief
introduction to string objects; you will learn more on objects and strings in Chapters 9 and 10.
4.2 Common Mathematical Functions
Key
Point
Java provides many useful methods in the Math class for performing common mathematical functions.
A method is a group of statements that performs a specific task. You have already used the
pow(a, b) method to compute ab in Section 2.9.4, Exponent Operations and the random()
method for generating a random number in Section 3.7. This section introduces other useful
methods in the Math class. They can be categorized as trigonometric methods, exponent methods,
and service methods. Service methods include the rounding, min, max, absolute, and random methods. In addition to methods, the Math class provides two useful double constants, PI and E (the
base of natural logarithms). You can use these constants as Math.PI and Math.E in any program.
4.2.1
VideoNote
Introduce math functions
Trigonometric Methods
The Math class contains the following methods as shown in Table 4.1 for performing
trigonometric functions:
TABLE 4.1 Trigonometric Methods in the Math Class
Method
Description
sin(radians)
Returns the trigonometric sine of an angle in radians.
cos(radians)
Returns the trigonometric cosine of an angle in radians.
tan(radians)
Returns the trigonometric tangent of an angle in radians.
toRadians(degree)
Returns the angle in radians for the angle in degree.
toDegree(radians)
Returns the angle in degrees for the angle in radians.
asin(a)
Returns the angle in radians for the inverse of sine.
acos(a)
Returns the angle in radians for the inverse of cosine.
atan(a)
Returns the angle in radians for the inverse of tangent.
4.2 Common Mathematical Functions 121
The parameter for sin, cos, and tan is an angle in radians. The return value for asin,
acos, and atan is a degree in radians in the range between -p/2 and p/2. One degree is
equal to p/180 in radians, 90 degrees is equal to p/2 in radians, and 30 degrees is equal to
p/6 in radians.
For example,
Math.toDegrees(Math.PI / 2) returns 90.0
Math.toRadians(30) returns 0.5236 (same as π/6)
Math.sin(0) returns 0.0
Math.sin(Math.toRadians(270)) returns -1.0
Math.sin(Math.PI / 6) returns 0.5
Math.sin(Math.PI / 2) returns 1.0
Math.cos(0) returns 1.0
Math.cos(Math.PI / 6) returns 0.866
Math.cos(Math.PI / 2) returns 0
Math.asin(0.5) returns 0.523598333 (same as π/6)
Math.acos(0.5) returns 1.0472 (same as π/3)
Math.atan(1.0) returns 0.785398 (same as π/4)
4.2.2
Exponent Methods
There are five methods related to exponents in the Math class as shown in Table 4.2.
TABLE 4.2 Exponent Methods in the Math Class
Method
Description
exp(x)
Returns e raised to power of x (ex).
log(x)
Returns the natural logarithm of x (ln(x) = loge(x)).
log10(x)
Returns the base 10 logarithm of x (log10(x)).
pow(a, b)
Returns a raised to the power of b (ab).
sqrt(x)
Returns the square root of x ( 2x) for x 7 = 0.
For example,
Math.exp(1) returns 2.71828
Math.log(Math.E) returns 1.0
Math.log10(10) returns 1.0
Math.pow(2, 3) returns 8.0
Math.pow(3, 2) returns 9.0
Math.pow(4.5, 2.5) returns 22.91765
Math.sqrt(4) returns 2.0
Math.sqrt(10.5) returns 4.24
4.2.3
The Rounding Methods
The Math class contains five rounding methods as shown in Table 4.3.
TABLE 4.3 Rounding Methods in the Math Class
Method
Description
ceil(x)
x is rounded up to its nearest integer. This integer is returned as a double value.
floor(x)
x is rounded down to its nearest integer. This integer is returned as a double value.
rint(x)
x is rounded up to its nearest integer. If x is equally close to two integers, the even one is returned as a double value.
round(x)
Returns (int)Math.floor(x + 0.5) if x is a float and returns (long)Math.floor(x + 0.5) if x is a double.
122 Chapter 4
Mathematical Functions, Characters, and Strings
For example,
Math.ceil(2.1) returns 4.0
Math.ceil(2.0) returns 2.0
Math.ceil(-2.0) returns -2.0
Math.ceil(-2.1) returns -2.0
Math.floor(2.1) returns 2.0
Math.floor(2.0) returns 2.0
Math.floor(-2.0) returns –2.0
Math.floor(-2.1) returns -4.0
Math.rint(2.1) returns 2.0
Math.rint(-2.0) returns –2.0
Math.rint(-2.1) returns -2.0
Math.rint(2.5) returns 2.0
Math.rint(4.5) returns 4.0
Math.rint(-2.5) returns -2.0
Math.round(2.6f) returns 3 // Returns int
Math.round(2.0) returns 2 // Returns long
Math.round(-2.0f) returns -2 // Returns int
Math.round(-2.6) returns -3 // Returns long
Math.round(-2.4) returns -2 // Returns long
4.2.4
The min, max, and abs Methods
The min and max methods return the minimum and maximum numbers of two numbers (int,
long, float, or double). For example, max(4.4, 5.0) returns 5.0, and min(3, 2)
returns 2.
The abs method returns the absolute value of the number (int, long, float, or double).
For example,
Math.max(2, 3) returns 3
Math.max(2.5, 3) returns 4.0
Math.min(2.5, 4.6) returns 2.5
Math.abs(-2) returns 2
Math.abs(-2.1) returns 2.1
4.2.5
The random Method
You have used the random() method in the preceding chapter. This method generates a random double value greater than or equal to 0.0 and less than 1.0 (0 <= Math.random() <
1.0). You can use it to write a simple expression to generate random numbers in any range.
For example,
(int)(Math.random() * 10)
50 + (int)(Math.random() * 50)
Returns a random integer
between 0 and 9.
Returns a random integer
between 50 and 99.
In general,
a + Math.random() * b
Returns a random number between a
and a + b, excluding a + b.
4.2 Common Mathematical Functions 123
4.2.6
Case Study: Computing Angles of a Triangle
You can use the math methods to solve many computational problems. Given the three sides
of a triangle, for example, you can compute the angles by using the following formula:
A = acos((a * a - b * b - c * c) / (-2 * b * c))
B = acos((b * b - a * a - c * c) / (-2 * a * c))
C = acos((c * c - b * b - a * a) / (-2 * a * b))
x2, y2
c
a
B
C
A
x3, y3
b
x1, y1
Don’t be intimidated by the mathematic formula. As we discussed early in Listing 2.9,
ComuteLoan.java, you don’t have to know how the mathematical formula is derived in order
to write a program for computing the loan payments. Here in this example, given the length of
three sides, you can use this formula to write a program to compute the angles without having
to know how the formula is derived. In order to compute the lengths of the sides, we need to
know the coordinates of three corner points and compute the distances between the points.
Listing 4.1 is an example of a program that prompts the user to enter the x- and y-coordinates
of the three corner points in a triangle and then displays the three angles.
LISTING 4.1 ComputeAngles.java
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
import java.util.Scanner;
public class ComputeAngles {
public static void main(String[] args) {
Scanner input = new Scanner(System.in);
// Prompt the user to enter three points
System.out.print("Enter three points: ");
double x1 = input.nextDouble();
double y1 = input.nextDouble();
double x2 = input.nextDouble();
double y2 = input.nextDouble();
double x3 = input.nextDouble();
double y3 = input.nextDouble();
// Compute three sides
double a = Math.sqrt((x2 - x3) * (x2 - x3)
+ (y2 - y3) * (y2 - y3));
double b = Math.sqrt((x1 - x3) * (x1 - x3)
+ (y1 - y3) * (y1 - y3));
double c = Math.sqrt((x1 - x2) * (x1 - x2)
+ (y1 - y2) * (y1 - y2));
enter three points
compute sides
// Compute three angles
double A = Math.toDegrees(Math.acos((a * a - b * b - c * c)
/ (-2 * b * c)));
double B = Math.toDegrees(Math.acos((b * b - a * a - c * c)
/ (-2 * a * c)));
double C = Math.toDegrees(Math.acos((c * c - b * b - a * a)
/ (-2 * a * b)));
// Display results
System.out.println("The three angles are " +
Math.round(A * 100) / 100.0 + " " +
display result
124 Chapter 4
Mathematical Functions, Characters, and Strings
35
36
37
38
Math.round(B * 100) / 100.0 + " " +
Math.round(C * 100) / 100.0);
}
}
Enter three points: 1 1 6.5 1 6.5 2.5
The three angles are 15.26 90.0 74.74
The program prompts the user to enter three points (line 8). This prompting message
is not clear. You should give the user explicit instructions on how to enter these points as
follows:
System.out.print("Enter the coordinates of three points separated "
+ "by spaces like x1 y1 x2 y2 x3 y3: ");
Note that the distance between two points (x1, y1) and (x2, y2) can be computed using the formula 2(x2 - x1)2 + (y2 - y1)2. The program computes the distances between two points (lines 17–22), and applies the formula to compute the angles
(lines 25–30). The angles are rounded to display up to two digits after the decimal point
(lines 34–36).
The Math class is used in the program, but not imported, because it is in the java.
lang package. All the classes in the java.lang package are implicitly imported in a Java
program.
✓
Check
Point
4.1
Evaluate the following method calls:
(a) Math.sqrt(4)
(b) Math.sin(2 * Math.PI)
(j) Math.floor(-2.5)
(k) Math.round(-2.5f)
(c) Math.cos(2 * Math.PI)
(d) Math.pow(2, 2)
(l) Math.round(-2.5)
(m) Math.rint(2.5)
(e)
(f)
(g)
(h)
(i)
(n)
(o)
(p)
(q)
(r)
Math.log(Math.E)
Math.exp(1)
Math.max(2, Math.min(3, 4))
Math.rint(-2.5)
Math.ceil(-2.5)
Math.ceil(2.5)
Math.floor(2.5)
Math.round(2.5f)
Math.round(2.5)
Math.round(Math.abs(-2.5))
4.2
4.3
True or false? The argument for trigonometric methods is an angle in radians.
Write a statement that converts 47 degrees to radians and assigns the result to a
variable.
4.4
Write a statement that converts π / 7 to an angle in degrees and assigns the result
to a variable.
Write an expression that obtains a random integer between 34 and 55. Write an
expression that obtains a random integer between 0 and 999. Write an expression
that obtains a random number between 5.5 and 55.5.
4.5
4.6
4.7
Why does the Math class not need to be imported?
What is Math.log(Math.exp(5.5))? What is Math.exp(Math.log(5.5))?
What is Math.asin(Math.sin(Math.PI / 6))? What is Math.sin(Math.
asin(Math.PI / 6))?
4.3 Character Data Type and Operations 125
4.3 Character Data Type and Operations
A character data type represents a single character.
In addition to processing numeric values, you can process characters in Java. The character
data type, char, is used to represent a single character. A character literal is enclosed in single
quotation marks. Consider the following code:
Key
Point
char type
char letter = 'A';
char numChar = '4';
The first statement assigns character A to the char variable letter. The second statement
assigns digit character 4 to the char variable numChar.
Caution
A string literal must be enclosed in quotation marks (" "). A character literal is a single
character enclosed in single quotation marks (' '). Therefore, "A" is a string, but 'A'
is a character.
4.3.1
char literal
Unicode and ASCII code
Computers use binary numbers internally. A character is stored in a computer as a sequence
of 0s and 1s. Mapping a character to its binary representation is called encoding. There are
different ways to encode a character. How characters are encoded is defined by an encoding
scheme.
Java supports Unicode, an encoding scheme established by the Unicode Consortium to
support the interchange, processing, and display of written texts in the world’s diverse languages. Unicode was originally designed as a 16-bit character encoding. The primitive data
type char was intended to take advantage of this design by providing a simple data type
that could hold any character. However, it turned out that the 65,536 characters possible in
a 16-bit encoding are not sufficient to represent all the characters in the world. The Unicode
standard therefore has been extended to allow up to 1,112,064 characters. Those characters
that go beyond the original 16-bit limit are called supplementary characters. Java supports
the supplementary characters. The processing and representing of supplementary characters
are beyond the scope of this book. For simplicity, this book considers only the original 16-bit
Unicode characters. These characters can be stored in a char type variable.
A 16-bit Unicode takes two bytes, preceded by \u, expressed in four hexadecimal digits that
run from \u0000 to \uFFFF. Hexadecimal numbers are introduced in Appendix F, Number
Systems. For example, the English word welcome is translated into Chinese using two characters,
. The Unicodes of these two characters are \u6B22\u8FCE. The Unicodes for the
Greek letters a b g are \u03b1 \u03b2 \u03b4.
Most computers use ASCII (American Standard Code for Information Interchange), an
8-bit encoding scheme for representing all uppercase and lowercase letters, digits, punctuation
marks, and control characters. Unicode includes ASCII code, with \u0000 to \u007F corresponding to the 128 ASCII characters. Table 4.4 shows the ASCII code for some commonly
used characters. Appendix B, ‘The ASCII Character Set,’ gives a complete list of ASCII
characters and their decimal and hexadecimal codes.
TABLE 4.4 ASCII Code for Commonly Used Characters
Characters
Code Value in Decimal
Unicode Value
'0' to '9'
48 to 57
\u0030 to \u0039
'A' to 'Z'
65 to 90
\u0041 to \u005A
'a' to 'z'
97 to 122
\u0061 to \u007A
encoding
Unicode
original Unicode
supplementary Unicode
126 Chapter 4
ASCII
Mathematical Functions, Characters, and Strings
You can use ASCII characters such as 'X', '1', and '$' in a Java program as well as
Unicodes. Thus, for example, the following statements are equivalent:
char letter = 'A';
char letter = '\u0041'; // Character A's Unicode is 0041
Both statements assign character A to the char variable letter.
Note
The increment and decrement operators can also be used on char variables to get the
next or preceding Unicode character. For example, the following statements display
character b.
char increment and
decrement
char ch = 'a';
System.out.println(++ch);
4.3.2
Escape Sequences for Special Characters
Suppose you want to print a message with quotation marks in the output. Can you write a
statement like this?
System.out.println("He said "Java is fun"");
escape sequence
No, this statement has a compile error. The compiler thinks the second quotation character
is the end of the string and does not know what to do with the rest of characters.
To overcome this problem, Java uses a special notation to represent special characters, as
shown in Table 4.5. This special notation, called an escape sequence, consists of a backslash
(\) followed by a character or a combination of digits. For example, \t is an escape sequence
for the Tab character and an escape sequence such as \u03b1 is used to represent a Unicode.
The symbols in an escape sequence are interpreted as a whole rather than individually. An
escape sequence is considered as a single character.
So, now you can print the quoted message using the following statement:
System.out.println("He said \"Java is fun\"");
The output is
He said "Java is fun"
Note that the symbols \ and " together represent one character.
TABLE 4.5 Escape Sequences
escape character
Escape Sequence
Name
Unicode Code
Decimal Value
\b
Backspace
\u0008
\t
Tab
\u0009
9
\n
Linefeed
\u000A
10
\f
Formfeed
\u000C
12
\r
Carriage Return
\u000D
13
\\
Backslash
\u005C
92
\"
Double Quote
\u0022
34
8
The backslash \ is called an escape character. It is a special character. To display this
character, you have to use an escape sequence \\. For example, the following code
System.out.println("\\t is a tab character");
displays
\t is a tab character
4.3 Character Data Type and Operations 127
4.3.3
Casting between char and Numeric Types
A char can be cast into any numeric type, and vice versa. When an integer is cast into a char,
only its lower 16 bits of data are used; the other part is ignored. For example:
char ch = (char)0XAB0041; // The lower 16 bits hex code 0041 is
// assigned to ch
System.out.println(ch);
// ch is character A
When a floating-point value is cast into a char, the floating-point value is first cast into an
int, which is then cast into a char.
char ch = (char)65.25;
System.out.println(ch);
// Decimal 65 is assigned to ch
// ch is character A
When a char is cast into a numeric type, the character’s Unicode is cast into the specified
numeric type.
int i = (int)'A'; // The Unicode of character A is assigned to i
System.out.println(i); // i is 65
Implicit casting can be used if the result of a casting fits into the target variable. Otherwise,
explicit casting must be used. For example, since the Unicode of 'a' is 97, which is within
the range of a byte, these implicit castings are fine:
byte b = 'a';
int i = 'a';
But the following casting is incorrect, because the Unicode \uFFF4 cannot fit into a byte:
byte b = '\uFFF4';
To force this assignment, use explicit casting, as follows:
byte b = (byte)'\uFFF4';
Any positive integer between 0 and FFFF in hexadecimal can be cast into a character
implicitly. Any number not in this range must be cast into a char explicitly.
All numeric operators can be applied to char operands. A char operand is automatically cast into a number if the other operand is a number or a character. If the other operand is a string, the character is concatenated with the string. For example, the following
statements
int i = '2' + '3'; // (int)'2' is 50 and (int)'3' is 51
System.out.println("i is " + i); // i is 101
int j = 2 + 'a'; // (int)'a' is 97
System.out.println("j is " + j); // j is 99
System.out.println(j + " is the Unicode for character "
+ (char)j); // 99 is the Unicode for character c
System.out.println("Chapter " + '2');
display
i is 101
j is 99
99 is the Unicode for character c
Chapter 2
numeric operators on
characters
128 Chapter 4
Mathematical Functions, Characters, and Strings
4.3.4
Comparing and Testing Characters
Two characters can be compared using the relational operators just like comparing two
numbers. This is done by comparing the Unicodes of the two characters. For example,
'a' < 'b' is true because the Unicode for 'a' (97) is less than the Unicode for 'b' (98).
'a' < 'A' is false because the Unicode for 'a' (97) is greater than the Unicode for 'A' (65).
'1' < '8' is true because the Unicode for '1' (49) is less than the Unicode for '8' (56).
Often in the program, you need to test whether a character is a number, a letter, an uppercase
letter, or a lowercase letter. As shown in Appendix B, the ASCII character set, that the Unicodes for lowercase letters are consecutive integers starting from the Unicode for 'a', then for
'b', 'c', . . ., and 'z'. The same is true for the uppercase letters and for numeric characters.
This property can be used to write the code to test characters. For example, the following code
tests whether a character ch is an uppercase letter, a lowercase letter, or a digital character.
if (ch >= 'A' && ch <= 'Z')
System.out.println(ch + "
else if (ch >= 'a' && ch <=
System.out.println(ch + "
else if (ch >= '0' && ch <=
System.out.println(ch + "
is an uppercase letter");
'z')
is a lowercase letter");
'9')
is a numeric character");
For convenience, Java provides the following methods in the Character class for testing
characters as shown in Table 4.6.
TABLE 4.6 Methods in the Character Class
Method
Description
isDigit(ch)
Returns true if the specified character is a digit.
isLetter(ch)
Returns true if the specified character is a letter.
isLetterOfDigit(ch)
Returns true if the specified character is a letter or digit.
isLowerCase(ch)
Returns true if the specified character is a lowercase letter.
isUpperCase(ch)
Returns true if the specified character is an uppercase letter.
toLowerCase(ch)
Returns the lowercase of the specified character.
toUpperCase(ch)
Returns the uppercase of the specified character.
For example,
System.out.println("isDigit('a') is " + Character.isDigit('a'));
System.out.println("isLetter('a') is " + Character.isLetter('a'));
System.out.println("isLowerCase('a') is "
+ Character.isLowerCase('a'));
System.out.println("isUpperCase('a') is "
+ Character.isUpperCase('a'));
System.out.println("toLowerCase('T') is "
+ Character.toLowerCase('T'));
System.out.println("toUpperCase('q') is "
+ Character.toUpperCase('q'));
displays
isDigit('a') is false
isLetter('a') is true
4.3 Character Data Type and Operations 129
isLowerCase('a')
isUpperCase('a')
toLowerCase('T')
toUpperCase('q')
is
is
is
is
true
false
t
Q
4.8
Use print statements to find out the ASCII code for '1', 'A', 'B', 'a', and 'b'.
Use print statements to find out the character for the decimal codes 40, 59, 79, 85,
and 90. Use print statements to find out the character for the hexadecimal code 40,
5A, 71, 72, and 7A.
4.9
Which of the following are correct literals for characters?
'1', '\u345dE', '\u3fFa', '\b', '\t'
4.10
4.11
How do you display the characters \ and "?
Evaluate the following:
int i = '1';
int j = '1' + '2' * ('4' - '3') + 'b' / 'a';
int k = 'a';
char c = 90;
4.12
Can the following conversions involving casting be allowed? If so, find the converted
result.
char c = 'A';
int i = (int)c;
float f = 1000.34f;
int i = (int)f;
double d = 1000.34;
int i = (int)d;
int i = 97;
char c = (char)i;
4.13
Show the output of the following program:
public class Test {
public static void main(String[] args) {
char x = 'a';
char y = 'c';
System.out.println(++x);
System.out.println(y++);
System.out.println(x - y);
}
}
4.14
4.15
Write the code that generates a random lowercase letter.
Show the output of the following statements:
System.out.println('a'
System.out.println('a'
System.out.println('a'
System.out.println('a'
System.out.println('a'
System.out.println('a'
< 'b');
<= 'A');
> 'b');
>= 'A');
== 'a');
!= 'b');
✓
Check
Point
130 Chapter 4
Mathematical Functions, Characters, and Strings
4.4 The String Type
Key
Point
VideoNote
Introduce strings and objects
A string is a sequence of characters.
The char type represents only one character. To represent a string of characters, use the data
type called String. For example, the following code declares message to be a string with
the value "Welcome to Java".
String message = "Welcome to Java";
String is a predefined class in the Java library, just like the classes System and Scanner.
The String type is not a primitive type. It is known as a reference type. Any Java class can
be used as a reference type for a variable. The variable declared by a reference type is known
as a reference variable that references an object. Here, message is a reference variable that
references a string object with contents Welcome to Java.
Reference data types will be discussed in detail in Chapter 9, Objects and Classes. For the
time being, you need to know only how to declare a String variable, how to assign a string
to the variable, and how to use the methods in the String class. More details on using strings
will be covered in Chapter 10.
Table 4.7 lists the String methods for obtaining string length, for accessing characters
in the string, for concatenating strings, for converting a string to upper or lowercases, and for
trimming a string.
TABLE 4.7 Simple Methods for String Objects
Method
Description
length()
Returns the number of characters in this string.
charAt(index)
Returns the character at the specified index from this string.
concat(s1)
Returns a new string that concatenates this string with string s1.
toUpperCase()
Returns a new string with all letters in uppercase.
toLowerCase()
Returns a new string with all letters in lowercase
trim()
Returns a new string with whitespace characters trimmed on both sides.
instance method
static method
Strings are objects in Java. The methods in Table 4.7 can only be invoked from a specific string instance. For this reason, these methods are called instance methods. A noninstance method is called a static method. A static method can be invoked without using
an object. All the methods defined in the Math class are static methods. They are not tied
to a specific object instance. The syntax to invoke an instance method is referenceVariable.methodName(arguments). A method may have many arguments or no arguments. For example, the charAt(index) method has one argument, but the length()
method has no arguments. Recall that the syntax to invoke a static method is ClassName
.methodName(arguments). For example, the pow method in the Math class can be invoked
using Math.pow(2, 2.5).
4.4.1
Getting String Length
You can use the length() method to return the number of characters in a string. For example, the following code
String message = "Welcome to Java";
System.out.println("The length of " + message + " is "
+ message.length());
4.4 The String Type 131
displays
The length of Welcome to Java is 15
Note
When you use a string, you often know its literal value. For convenience, Java allows
you to use the string literal to refer directly to strings without creating new variables.
Thus, "Welcome to Java".length() is correct and returns 15. Note that ""
denotes an empty string and "".length() is 0.
4.4.2
string literal
empty string
Getting Characters from a String
The s.charAt(index) method can be used to retrieve a specific character in a string s,
where the index is between 0 and s.length()–1. For example, message.charAt(0)
returns the character W, as shown in Figure 4.1. Note that the index for the first character in
the string is 0.
Indices
message
0
1
2
3
4
5
6
W
e
l
c
o
m
e
message.charAt(0)
FIGURE 4.1
7
8
9
t
o
charAt(index)
10 11 12 13 14
message.length() is 15
J
a
v
a
message.charAt(14)
The characters in a String object can be accessed using its index.
Caution
Attempting to access characters in a string s out of bounds is a common programming error. To avoid it, make sure that you do not use an index beyond
s.length() – 1. For example, s.charAt(s.length()) would cause a
StringIndexOutOfBoundsException.
4.4.3
string index range
Concatenating Strings
You can use the concat method to concatenate two strings. The statement shown below, for
example, concatenates strings s1 and s2 into s3:
String s3 = s1.concat(s2);
s1.concat(s2)
Because string concatenation is heavily used in programming, Java provides a convenient
way to accomplish it. You can use the plus (+) operator to concatenate two strings, so the
previous statement is equivalent to
String s3 = s1 + s2;
s1 + s2
The following code combines the strings message, " and ", and "HTML" into one string:
String myString = message + " and " + "HTML";
Recall that the + operator can also concatenate a number with a string. In this case, the
number is converted into a string and then concatenated. Note that at least one of the operands
must be a string in order for concatenation to take place. If one of the operands is a nonstring
concatenate strings and
numbers
132 Chapter 4
Mathematical Functions, Characters, and Strings
(e.g., a number), the nonstring value is converted into a string and concatenated with the other
string. Here are some examples:
// Three strings are concatenated
String message = "Welcome " + "to " + "Java";
// String Chapter is concatenated with number 2
String s = "Chapter" + 2; // s becomes Chapter2
// String Supplement is concatenated with character B
String s1 = "Supplement" + 'B'; // s1 becomes SupplementB
If neither of the operands is a string, the plus sign (+) is the addition operator that adds two
numbers.
The augmented += operator can also be used for string concatenation. For example, the
following code appends the string "and Java is fun" with the string "Welcome to
Java" in message.
message += " and Java is fun";
So the new message is "Welcome to Java and Java is fun".
If i = 1 and j = 2, what is the output of the following statement?
System.out.println("i + j is " + i + j);
The output is "i + j is 12" because "i + j is " is concatenated with the value of
i first. To force i + j to be executed first, enclose i + j in the parentheses, as follows:
System.out.println("i + j is " + (i + j));
4.4.4
Converting Strings
The toLowerCase() method returns a new string with all lowercase letters and the
toUpperCase() method returns a new string with all uppercase letters. For example,
toLowerCase()
toUpperCase()
whitespace character
trim()
"Welcome".toLowerCase() returns a new string welcome.
"Welcome".toUpperCase() returns a new string WELCOME.
The trim() method returns a new string by eliminating whitespace characters from both
ends of the string. The characters ' ', \t, \f, \r, or \n are known as whitespace characters.
For example,
"\t Good Night \n".trim() returns a new string Good Night.
4.4.5
read strings
Reading a String from the Console
To read a string from the console, invoke the next() method on a Scanner object. For
example, the following code reads three strings from the keyboard:
Scanner input = new Scanner(System.in);
System.out.print("Enter three words separated by spaces: ");
String s1 = input.next();
String s2 = input.next();
String s3 = input.next();
System.out.println("s1 is " + s1);
System.out.println("s2 is " + s2);
System.out.println("s3 is " + s3);
4.4 The String Type 133
Enter
s1 is
s2 is
s3 is
three words separated by spaces: Welcome to Java
Welcome
to
Java
The next() method reads a string that ends with a whitespace character. You can use
the nextLine() method to read an entire line of text. The nextLine() method reads a
string that ends with the Enter key pressed. For example, the following statements read a
line of text.
whitespace character
Scanner input = new Scanner(System.in);
System.out.println("Enter a line: ");
String s = input.nextLine();
System.out.println("The line entered is " + s);
Enter a line: Welcome to Java
The line entered is Welcome to Java
Important Caution
To avoid input errors, do not use nextLine() after nextByte(), nextShort(),
nextInt(), nextLong(), nextFloat(), nextDouble(), or next(). The
reasons will be explained in Section 12.11.4, ‘How Does Scanner Work?’
4.4.6
avoid input errors
Reading a Character from the Console
To read a character from the console, use the nextLine() method to read a string and then
invoke the charAt(0) method on the string to return a character. For example, the following
code reads a character from the keyboard:
Scanner input = new Scanner(System.in);
System.out.print("Enter a character: ");
String s = input.nextLine();
char ch = s.charAt(0);
System.out.println("The character entered is " + ch);
4.4.7
Comparing Strings
The String class contains the methods as shown in Table 4.8 for comparing two strings.
TABLE 4.8 Comparison Methods for String Objects
Method
Description
equals(s1)
Returns true if this string is equal to string s1.
equalsIgnoreCase(s1)
Returns true if this string is equal to string s1; it is case insensitive.
compareTo(s1)
Returns an integer greater than 0, equal to 0, or less than 0 to indicate whether this string is greater
than, equal to, or less than s1.
compareToIgnoreCase(s1)
Same as compareTo except that the comparison is case insensitive.
startsWith(prefix)
Returns true if this string starts with the specified prefix.
endsWith(suffix)
Returns true if this string ends with the specified suffix.
contains(s1)
Returns true if s1 is a substring in this string.
134 Chapter 4
Mathematical Functions, Characters, and Strings
How do you compare the contents of two strings? You might attempt to use the == operator,
as follows:
==
if (string1 == string2)
System.out.println("string1 and string2 are the same object");
else
System.out.println("string1 and string2 are different objects");
However, the == operator checks only whether string1 and string2 refer to the same
object; it does not tell you whether they have the same contents. Therefore, you cannot use the
== operator to find out whether two string variables have the same contents. Instead, you should
use the equals method. The following code, for instance, can be used to compare two strings:
string1.equals(string2)
if (string1.equals(string2))
System.out.println("string1 and string2 have the same contents");
else
System.out.println("string1 and string2 are not equal");
For example, the following statements display true and then false.
String s1 = "Welcome to Java";
String s2 = "Welcome to Java";
String s3 = "Welcome to C++";
System.out.println(s1.equals(s2)); // true
System.out.println(s1.equals(s3)); // false
The compareTo method can also be used to compare two strings. For example, consider
the following code:
s1.compareTo(s2)
s1.compareTo(s2)
The method returns the value 0 if s1 is equal to s2, a value less than 0 if s1 is lexicographically (i.e., in terms of Unicode ordering) less than s2, and a value greater than 0 if s1
is lexicographically greater than s2.
The actual value returned from the compareTo method depends on the offset of the first
two distinct characters in s1 and s2 from left to right. For example, suppose s1 is abc and s2
is abg, and s1.compareTo(s2) returns -4. The first two characters (a vs. a) from s1 and
s2 are compared. Because they are equal, the second two characters (b vs. b) are compared.
Because they are also equal, the third two characters (c vs. g) are compared. Since the character c is 4 less than g, the comparison returns -4.
Caution
Syntax errors will occur if you compare strings by using relational operators >, >=, <, or
<=. Instead, you have to use s1.compareTo(s2).
Note
The equals method returns true if two strings are equal and false if they are not.
The compareTo method returns 0, a positive integer, or a negative integer, depending
on whether one string is equal to, greater than, or less than the other string.
The String class also provides the equalsIgnoreCase and compareToIgnoreCase methods for comparing strings. The equalsIgnoreCase and compareToIgnoreCase methods ignore the case of the letters when comparing two strings. You can also
use str.startsWith(prefix) to check whether string str starts with a specified prefix,
str.endsWith(suffix) to check whether string str ends with a specified suffix, and str
.contains(s1) to check whether string str contains string s1 . For example,
"Welcome to Java".startsWith("We") returns true.
"Welcome to Java".startsWith("we") returns false.
"Welcome to Java".endsWith("va") returns true.
4.4 The String Type 135
"Welcome to Java".endsWith("v") returns false.
"Welcome to Java".contains("to") returns true.
"Welcome to Java".contains("To") returns false.
Listing 4.2 gives a program that prompts the user to enter two cities and displays them in
alphabetical order.
LISTING 4.2 OrderTwoCities.java
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
import java.util.Scanner;
public class OrderTwoCities {
public static void main(String[] args) {
Scanner input = new Scanner(System.in);
// Prompt the user to enter two cities
System.out.print("Enter the first city: ");
String city1 = input.nextLine();
System.out.print("Enter the second city: ");
String city2 = input.nextLine();
if (city1.compareTo(city2) < 0)
System.out.println("The cities in alphabetical order are " +
city1 + " " + city2);
else
System.out.println("The cities in alphabetical order are " +
city2 + " " + city1);
input city1
input city2
compare two cities
}
}
Enter the first city: New York
Enter the second city: Boston
The cities in alphabetical order are Boston New York
The program reads two strings for two cities (lines 9, 11). If input.nextLine() is replaced
by input.next() (line 9), you cannot enter a string with spaces for city1. Since a city name
may contain multiple words separated by spaces, the program uses the nextLine method to
read a string (lines 9, 11). Invoking city1.compareTo(city2) compares two strings city1
with city2 (line 13). A negative return value indicates that city1 is less than city2.
4.4.8
Obtaining Substrings
You can obtain a single character from a string using the charAt method. You can also
obtain a substring from a string using the substring method in the String class, as shown
in Table 4.9.
For example,
String message = "Welcome to Java";
String message = message.substring(0, 11) + "HTML";
The string message now becomes Welcome to HTML.
TABLE 4.9 The String class contains the methods for obtaining substrings.
Method
Description
substring(beginIndex)
Returns this string’s substring that begins with the character at the specified beginIndex and extends
to the end of the string, as shown in Figure 4.2.
substring(beginIndex,
Returns this string’s substring that begins at the specified beginIndex and extends to the character at index
endIndex – 1, as shown in Figure 4.2. Note that the character at endIndex is not part of the substring.
endIndex)
136 Chapter 4
Mathematical Functions, Characters, and Strings
Indices
Message
0
1
2
3
4
5
6
W
e
l
c
o
m
e
7
8
9
t
o
message.substring(0, 11)
FIGURE 4.2
10 11 12 13 14
J
a
v
a
message.substring(11)
The substring method obtains a substring from a string.
Note
beginIndex <= endIndex
If beginIndex is endIndex, substring(beginIndex, endIndex) returns an
empty string with length 0. If beginIndex > endIndex, it would be a runtime error.
4.4.9
Finding a Character or a Substring in a String
The String class provides several versions of indexOf and lastIndexOf methods to find
a character or a substring in a string, as shown in Table 4.10.
TABLE 4.10 The String class contains the methods for finding substrings.
Method
Description
index(ch)
Returns the index of the first occurrence of ch in the string. Returns -1 if not matched.
indexOf(ch, fromIndex)
Returns the index of the first occurrence of ch after fromIndex in the string. Returns -1 if not matched.
indexOf(s)
Returns the index of the first occurrence of string s in this string. Returns -1 if not matched.
indexOf(s, fromIndex)
Returns the index of the first occurrence of string s in this string after fromIndex. Returns -1 if not
matched.
lastIndexOf(ch)
Returns the index of the last occurrence of ch in the string. Returns -1 if not matched.
lastIndexOf(ch, fromIndex)
Returns the index of the last occurrence of ch before fromIndex in this string. Returns -1 if not
matched.
lastIndexOf(s)
Returns the index of the last occurrence of string s. Returns -1 if not matched.
lastIndexOf(s, fromIndex)
Returns the index of the last occurrence of string s before fromIndex. Returns -1 if not matched.
For example,
indexOf
"Welcome
"Welcome
"Welcome
"Welcome
"Welcome
"Welcome
to
to
to
to
to
to
Java".indexOf('W') returns 0.
Java".indexOf('o') returns 4.
Java".indexOf('o', 5) returns 9.
Java".indexOf("come") returns 3.
Java".indexOf("Java", 5) returns 11.
Java".indexOf("java", 5) returns -1.
lastIndexOf
"Welcome
"Welcome
"Welcome
"Welcome
"Welcome
"Welcome
to
to
to
to
to
to
Java".lastIndexOf('W') returns 0.
Java".lastIndexOf('o') returns 9.
Java".lastIndexOf('o', 5) returns 4.
Java".lastIndexOf("come") returns 3.
Java".lastIndexOf("Java", 5) returns -1.
Java".lastIndexOf("Java") returns 11.
Suppose a string s contains the first name and last name separated by a space. You can use the
following code to extract the first name and last name from the string:
int k = s.indexOf(' ');
String firstName = s.substring(0, k);
String lastName = s.substring(k + 1);
4.4 The String Type 137
For example, if s is Kim Jones, the following diagram illustrates how the first name and last
name are extracted.
Indices
Message
0
1
2
K
i
m
3
4
5
6
7
8
J
o
n
e
s
k is 3
s.substring
(0, k) is Kim
4.4.10
s.substring
(k + 1) is Jones
Conversion between Strings and Numbers
You can convert a numeric string into a number. To convert a string into an int value, use the
Integer.parseInt method, as follows:
Integer.parseInt method
int intValue = Integer.parseInt(intString);
where intString is a numeric string such as "123".
To convert a string into a double value, use the Double.parseDouble method, as
follows:
Double.parseDouble
method
double doubleValue = Double.parseDouble(doubleString);
where doubleString is a numeric string such as "123.45".
If the string is not a numeric string, the conversion would cause a runtime error. The
Integer and Double classes are both included in the java.lang package, and thus they are
automatically imported.
You can convert a number into a string, simply use the string concatenating operator as
follows:
String s = number + "";
4.16
number to string
Suppose that s1, s2, and s3 are three strings, given as follows:
String s1 = "Welcome to Java";
String s2 = "Programming is fun";
String s3 = "Welcome to Java";
What are the results of the following expressions?
(a)
(b)
(c)
(d)
(e)
(f)
(g)
(h)
(i)
(j)
s1 == s2
s2 == s3
s1.equals(s2)
s1.equals(s3)
s1.compareTo(s2)
s2.compareTo(s3)
s2.compareTo(s2)
s1.charAt(0)
s1.indexOf('j')
s1.indexOf("to")
(k) s1.lastIndexOf('a')
(l)
(m)
(n)
(o)
(p)
(q)
(r)
(s)
(t)
(u)
s1.lastIndexOf("o", 15)
s1.length()
s1.substring(5)
s1.substring(5, 11)
s1.startsWith("Wel")
s1.endsWith("Java")
s1.toLowerCase()
s1.toUpperCase()
s1.concat(s2)
s1.contains(s2)
(v) "\t Wel \t".trim()
✓
Check
Point
138 Chapter 4
Mathematical Functions, Characters, and Strings
4.17
Suppose that s1 and s2 are two strings. Which of the following statements or
expressions are incorrect?
String s = "Welcome to Java";
String s3 = s1 + s2;
String s3 = s1 - s2;
s1 == s2;
s1 >= s2;
s1.compareTo(s2);
int i = s1.length();
char c = s1(0);
char c = s1.charAt(s1.length());
4.18
Show the output of the following statements (write a program to verify your results):
System.out.println("1"
System.out.println('1'
System.out.println("1"
System.out.println("1"
System.out.println('1'
4.19
1);
1);
1 + 1);
(1 + 1));
1 + 1);
Evaluate the following expressions (write a program to verify your results):
1
1
1
1
4.20
+
+
+
+
+
+
+
+
+
"Welcome
"Welcome
"Welcome
"Welcome
"
"
"
"
+
+
+
+
1 + 1
(1 + 1)
('\u0001' + 1)
'a' + 1
Let s1 be " Welcome " and s2 be " welcome ". Write the code for the following
statements:
(a) Check whether s1 is equal to s2 and assign the result to a Boolean variable
isEqual.
(b) Check whether s1 is equal to s2, ignoring case, and assign the result to a
Boolean variable isEqual.
(c) Compare s1 with s2 and assign the result to an int variable x.
(d) Compare s1 with s2, ignoring case, and assign the result to an int
variable x.
(e) Check whether s1 has the prefix AAA and assign the result to a Boolean
variable b.
(f) Check whether s1 has the suffix AAA and assign the result to a Boolean
variable b.
(g) Assign the length of s1 to an int variable x.
(h) Assign the first character of s1 to a char variable x.
(i) Create a new string s3 that combines s1 with s2.
(j) Create a substring of s1 starting from index 1.
(k) Create a substring of s1 from index 1 to index 4.
(l) Create a new string s3 that converts s1 to lowercase.
(m) Create a new string s3 that converts s1 to uppercase.
(n) Create a new string s3 that trims whitespace characters on both ends of s1.
4.5 Case Studies 139
(o) Assign the index of the first occurrence of the character e in s1 to an int
variable x.
(p) Assign the index of the last occurrence of the string abc in s1 to an int
variable x.
4.5 Case Studies
Strings are fundamental in programming. The ability to write programs using strings
is essential in learning Java programming.
Key
Point
You will frequently use strings to write useful programs. This section presents three examples
of solving problems using strings.
4.5.1
Case Study: Guessing Birthdays
You can find out the date of the month when your friend was born by asking five questions.
Each question asks whether the day is in one of the five sets of numbers.
= 19
+
3 5 7
1
9 11 13 15
17 19 21 23
25 27 29 31
2 3 6 7
10 11 14 15
18 19 22 23
26 27 30 31
4 5 6 7
12 13 14 15
20 21 22 23
28 29 30 31
8 9 10 11
12 13 14 15
24 25 26 27
28 29 30 31
Set1
Set2
Set3
Set4
16
20
24
28
17
21
25
29
18
22
26
30
19
23
27
31
Set5
The birthday is the sum of the first numbers in the sets where the day appears. For example,
if the birthday is 19, it appears in Set1, Set2, and Set5. The first numbers in these three sets
are 1, 2, and 16. Their sum is 19.
Listing 4.3 gives a program that prompts the user to answer whether the day is in Set1
(lines 41–44), in Set2 (lines 50–53), in Set3 (lines 59–62), in Set4 (lines 68–71), and in Set5
(lines 77–80). If the number is in the set, the program adds the first number in the set to day
(lines 47, 56, 65, 74, 83).
LISTING 4.3 GuessBirthday.java
1
2
3
4
5
6
7
8
9
10
11
12
import java.util.Scanner;
public class GuessBirthday {
public static void main(String[] args) {
String set1 =
" 1 3 5 7\n" +
" 9 11 13 15\n" +
"17 19 21 23\n" +
"25 27 29 31";
String set2 =
" 2 3 6 7\n" +
140 Chapter 4
day to be determined
in Set1?
in Set2?
in Set3?
Mathematical Functions, Characters, and Strings
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
"10 11 14 15\n" +
"18 19 22 23\n" +
"26 27 30 31";
String set3
" 4 5 6
"12 13 14
"20 21 22
"28 29 30
=
String set4
" 8 9 10
"12 13 14
"24 25 26
"28 29 30
=
11\n" +
15\n" +
27\n" +
31";
String set5
"16 17 18
"20 21 22
"24 25 26
"28 29 30
=
19\n" +
23\n" +
27\n" +
31";
7\n" +
15\n" +
23\n" +
31";
int day = 0;
// Create a Scanner
Scanner input = new Scanner(System.in);
// Prompt the user to answer questions
System.out.print("Is your birthday in Set1?\n");
System.out.print(set1);
System.out.print("\nEnter 0 for No and 1 for Yes: ");
int answer = input.nextInt();
if (answer == 1)
day += 1;
// Prompt the user to answer questions
System.out.print("\nIs your birthday in Set2?\n");
System.out.print(set2);
System.out.print("\nEnter 0 for No and 1 for Yes: ");
answer = input.nextInt();
if (answer == 1)
day += 2;
// Prompt the user to answer questions
System.out.print("Is your birthday in Set3?\n");
System.out.print(set3);
System.out.print("\nEnter 0 for No and 1 for Yes: ");
answer = input.nextInt();
if (answer == 1)
day += 4;
// Prompt the user to answer questions
System.out.print("\nIs your birthday in Set4?\n");
System.out.print(set4);
System.out.print("\nEnter 0 for No and 1 for Yes: ");
answer = input.nextInt();
4.5 Case Studies 141
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
if (answer == 1)
day += 8;
in Set4?
// Prompt the user to answer questions
System.out.print("\nIs your birthday in Set5?\n");
System.out.print(set5);
System.out.print("\nEnter 0 for No and 1 for Yes: ");
answer = input.nextInt();
if (answer == 1)
day += 16;
System.out.println("\nYour birthday is " + day + "!");
}
}
Is your birthday in Set1?
1 3 5 7
9 11 13 15
17 19 21 23
25 27 29 31
Enter 0 for No and 1 for Yes: 1
Is your birthday in Set2?
2 3 6 7
10 11 14 15
18 19 22 23
26 27 30 31
Enter 0 for No and 1 for Yes: 1
Is your birthday in Set3?
4 5 6 7
12 13 14 15
20 21 22 23
28 29 30 31
Enter 0 for No and 1 for Yes: 0
Is your birthday in Set4?
8 9 10 11
12 13 14 15
24 25 26 27
28 29 30 31
Enter 0 for No and 1 for Yes: 0
Is your birthday in Set5?
16 17 18 19
20 21 22 23
24 25 26 27
28 29 30 31
Enter 0 for No and 1 for Yes: 1
Your birthday is 19!
in Set5?
142 Chapter 4
Mathematical Functions, Characters, and Strings
line#
35
day
answer
0
44
47
1
1
53
56
1
3
62
0
71
0
80
1
83
output
19
85
mathematics behind the game
Your birthday is 19!
This game is easy to program. You may wonder how the game was created. The mathematics
behind the game is actually quite simple. The numbers are not grouped together by accident—
the way they are placed in the five sets is deliberate. The starting numbers in the five sets are
1, 2, 4, 8, and 16, which correspond to 1, 10, 100, 1000, and 10000 in binary (binary numbers are introduced in Appendix F, Number Systems). A binary number for decimal integers
between 1 and 31 has at most five digits, as shown in Figure 4.3a. Let it be b5b4b3b2b1. Thus,
b5b4b3b2b1 = b50000 + b4000 + b300 + b20 + b1,as shown in Figure 4.3b. If a day’s binary
number has a digit 1 in bk, the number should appear in Setk. For example, number 19 is
binary 10011, so it appears in Set1, Set2, and Set5. It is binary 1 + 10 + 10000 = 10011
or decimal 1 + 2 + 16 = 19. Number 31 is binary 11111, so it appears in Set1, Set2, Set3,
Set4, and Set5. It is binary 1 + 10 + 100 + 1000 + 10000 = 11111 or decimal 1 + 2 +
4 + 8 + 16 = 31.
Decimal
1
2
3
...
19
...
31
Binary
00001
00010
00011
10011
11111
(a)
b5 0 0 0
b4 0 0
b3 0
b2
+
0
0
0
0
b1
10000
10
+
1
10011
b5 b 4 b 3 b 2 b 1
19
10000
1000
100
10
+
1
11111
31
(b)
FIGURE 4.3 (a) A number between 1 and 31 can be represented using a five-digit binary
number. (b) A five-digit binary number can be obtained by adding binary numbers 1, 10,
100, 1000, or 10000.
✓
Check
Point
4.21
If you run Listing 4.3 GuessBirthday.java with input 1 for Set1, Set3, and Set4 and 0
for Set2 and Set5, what will be the birthday?
4.5.2 Case Study: Converting a Hexadecimal Digit to a Decimal Value
The hexadecimal number system has 16 digits: 0–9, A–F. The letters A, B, C, D, E, and F
correspond to the decimal numbers 10, 11, 12, 13, 14, and 15. We now write a program that
prompts the user to enter a hex digit and display its corresponding decimal value, as shown
in Listing 4.4.
4.5 Case Studies 143
LISTING 4.4 HexDigit2Dec.java
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
import java.util.Scanner;
VideoNote
Convert hex to decimal
public class HexDigit2Dec {
public static void main(String[] args) {
Scanner input = new Scanner(System.in);
System.out.print("Enter a hex digit: ");
String hexString = input.nextLine();
// Check if the hex string has exactly one character
if (hexString.length() != 1) {
System.out.println("You must enter exactly one character");
System.exit(1);
}
// Display decimal value for the hex digit
char ch = hexString.charAt(0);
if (ch <= 'F' && ch >= 'A') {
int value = ch - 'A' + 10;
System.out.println("The decimal value for hex digit "
+ ch + " is " + value);
}
else if (Character.isDigit(ch)) {
System.out.println("The decimal value for hex digit "
+ ch + " is " + ch);
}
else {
System.out.println(ch + " is an invalid input");
}
}
}
Enter a hex digit: AB7C
You must enter exactly one character
Enter a hex digit: B
The decimal value for hex digit B is 11
Enter a hex digit: 8
The decimal value for hex digit 8 is 8
Enter a hex digit: T
T is an invalid input
The program reads a string from the console (line 7) and checks if the string contains a
single character (line 10). If not, report an error and exit the program (line 12).
The program invokes the Character.toUpperCase method to obtain the character ch as
an uppercase letter (line 16). If ch is between 'A' and 'F' (line 17), the corresponding decimal value is ch – 'A' + 10 (line 18). Note that ch – 'A' is 0 if ch is 'A', ch – 'A' is 1
input string
check length
is A-F?
is 0-9?
144 Chapter 4
Mathematical Functions, Characters, and Strings
if ch is 'B', and so on. When two characters perform a numerical operation, the characters’
Unicodes are used in the computation.
The program invokes the Character.isDigit(ch) method to check if ch is between
'0' and '9' (line 22). If so, the corresponding decimal digit is the same as ch (lines 23–24).
If ch is not between 'A' and 'F' nor a digit character, the program displays an error message
(line 27).
4.5.3
Case Study: Revising the Lottery Program Using Strings
The lottery program in Listing 3.8, Lottery.java, generates a random two-digit number, prompts
the user to enter a two-digit number, and determines whether the user wins according to the
following rule:
1. If the user input matches the lottery number in the exact order, the award is $10,000.
2. If all the digits in the user input match all the digits in the lottery number, the award is
$3,000.
3. If one digit in the user input matches a digit in the lottery number, the award is $1,000.
The program in Listing 3.8 uses an integer to store the number. Listing 4.5 gives a new
program that generates a random two-digit string instead of a number and receives the user
input as a string instead of a number.
LISTING 4.5 LotteryUsingStrings.java
generate a lottery
enter a guess
exact match?
match all digits?
match one digit?
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
import java.util.Scanner;
public class LotteryUsingStrings {
public static void main(String[] args) {
// Generate a lottery as a two-digit string
String lottery = "" + (int)(Math.random() * 10)
+ (int)(Math.random() * 10);
// Prompt the user to enter a guess
Scanner input = new Scanner(System.in);
System.out.print("Enter your lottery pick (two digits): ");
String guess = input.nextLine();
// Get digits from lottery
char lotteryDigit1 = lottery.charAt(0);
char lotteryDigit2 = lottery.charAt(1);
// Get digits from guess
char guessDigit1 = guess.charAt(0);
char guessDigit2 = guess.charAt(1);
System.out.println("The lottery number is " + lottery);
// Check the guess
if (guess.equals(lottery))
System.out.println("Exact match: you win $10,000");
else if (guessDigit2 == lotteryDigit1
&& guessDigit1 == lotteryDigit2)
System.out.println("Match all digits: you win $3,000");
else if (guessDigit1 == lotteryDigit1
|| guessDigit1 == lotteryDigit2
|| guessDigit2 == lotteryDigit1
|| guessDigit2 == lotteryDigit2)
System.out.println("Match one digit: you win $1,000");
4.6 Formatting Console Output 145
35
36
37
38
else
System.out.println("Sorry, no match");
}
}
Enter your lottery pick (two digits): 00
The lottery number is 00
Exact match: you win $10,000
Enter your lottery pick (two digits): 45
The lottery number is 54
Match all digits: you win $3,000
Enter your lottery pick: 23
The lottery number is 34
Match one digit: you win $1,000
Enter your lottery pick: 23
The lottery number is 14
Sorry: no match
The program generates two random digits and concatenates them into the string lottery
(lines 6–7). After this, lottery contains two random digits.
The program prompts the user to enter a guess as a two-digit string (line 12) and checks the
guess against the lottery number in this order:
■
First check whether the guess matches the lottery exactly (line 25).
■
If not, check whether the reversal of the guess matches the lottery (line 27).
■
If not, check whether one digit is in the lottery (lines 30–33).
■
If not, nothing matches and display “Sorry, no match” (line 36).
4.6 Formatting Console Output
You can use the System.out.printf method to display formatted output on the
console.
Often, it is desirable to display numbers in a certain format. For example, the following code
computes interest, given the amount and the annual interest rate.
double amount = 12618.98;
double interestRate = 0.0013;
double interest = amount * interestRate;
System.out.println("Interest is $" + interest);
Interest is $16.404674
Key
Point
146 Chapter 4
Mathematical Functions, Characters, and Strings
Because the interest amount is currency, it is desirable to display only two digits after the
decimal point. To do this, you can write the code as follows:
double amount = 12618.98;
double interestRate = 0.0013;
double interest = amount * interestRate;
System.out.println("Interest is $"
+ (int)(interest * 100) / 100.0);
Interest is $16.4
However, the format is still not correct. There should be two digits after the decimal point:
printf
16.40 rather than 16.4. You can fix it by using the printf method, like this:
double amount = 12618.98;
double interestRate = 0.0013;
double interest = amount * interestRate;
System.out.printf("Interest is $%4.2f",
interest);
% 4 . 2 f
field width
format specifier
conversion code
precision
Interest is $16.40
The syntax to invoke this method is
System.out.printf(format, item1, item2, ..., itemk)
format specifier
where format is a string that may consist of substrings and format specifiers.
A format specifier specifies how an item should be displayed. An item may be a numeric
value, a character, a Boolean value, or a string. A simple format specifier consists of a percent
sign (%) followed by a conversion code. Table 4.11 lists some frequently used simple format
specifiers.
TABLE 4.11 Frequently Used Format Specifiers
Format Specifier
Output
Example
%b
a Boolean value
true or false
%c
a character
‘a’
%d
a decimal integer
200
%f
a floating-point number
45.460000
%e
a number in standard scientific notation
4.556000e + 01
%s
a string
“Java is cool”
Here is an example:
items
int count = 5;
double amount = 45.56;
System.out.printf("count is %d and amount is %f", count, amount);
display
count is 5 and amount is 45.560000
4.6 Formatting Console Output 147
Items must match the format specifiers in order, in number, and in exact type. For example,
the format specifier for count is %d and for amount is %f. By default, a floating-point value
is displayed with six digits after the decimal point. You can specify the width and precision in
a format specifier, as shown in the examples in Table 4.12.
TABLE 4.12 Examples of Specifying Width and Precision
Example
Output
%5c
Output the character and add four spaces before the character item, because the
width is 5.
%6b
Output the Boolean value and add one space before the false value and two spaces
before the true value.
%5d
Output the integer item with width at least 5. If the number of digits in the item is
6 5, add spaces before the number. If the number of digits in the item is 7 5, the
width is automatically increased.
%10.2f
Output the floating-point item with width at least 10 including a decimal point
and two digits after the point. Thus, there are 7 digits allocated before the decimal
point. If the number of digits before the decimal point in the item is 6 7, add spaces
before the number. If the number of digits before the decimal point in the item is
7 7, the width is automatically increased.
%10.2e
Output the floating-point item with width at least 10 including a decimal point, two
digits after the point and the exponent part. If the displayed number in scientific
notation has width less than 10, add spaces before the number.
%12s
Output the string with width at least 12 characters. If the string item has fewer
than 12 characters, add spaces before the string. If the string item has more than
12 characters, the width is automatically increased.
If an item requires more spaces than the specified width, the width is automatically
increased. For example, the following code
System.out.printf("%3d#%2s#%4.2f\n", 1234, "Java", 51.6653);
displays
1234#Java#51.67
The specified width for int item 1234 is 3, which is smaller than its actual size 4. The
width is automatically increased to 4. The specified width for string item Java is 2, which is
smaller than its actual size 4. The width is automatically increased to 4. The specified width
for double item 51.6653 is 4, but it needs width 5 to display 51.67, so the width is automatically increased to 5.
By default, the output is right justified. You can put the minus sign (-) in the format
specifier to specify that the item is left justified in the output within the specified field. For
example, the following statements
System.out.printf("%8d%8s%8.1f\n", 1234, "Java", 5.63);
System.out.printf("%-8d%-8s%-8.1f \n", 1234, "Java", 5.63);
display
8
8
8
1234
Java
5.6
1234
Java
5.6
where the square box (n) denotes a blank space.
right justify
left justify
148 Chapter 4
Mathematical Functions, Characters, and Strings
Caution
The items must match the format specifiers in exact type. The item for the format
specifier %f or %e must be a floating-point type value such as 40.0, not 40. Thus, an
int variable cannot match %f or %e.
Tip
The % sign denotes a format specifier. To output a literal % in the format string, use %%.
Listing 4.6 gives a program that uses printf to display a table.
LISTING 4.6 FormatDemo.java
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
display table header
values for 30 degrees
values for 60 degrees
public class FormatDemo {
public static void main(String[] args) {
// Display the header of the table
System.out.printf("%-10s%-10s%-10s%-10s%-10s\n", "Degrees",
"Radians", "Sine", "Cosine", "Tangent");
// Display values for 30 degrees
int degrees = 30;
double radians = Math.toRadians(degrees);
System.out.printf("%-10d%-10.4f%-10.4f%-10.4f%-10.4f\n", degrees,
radians, Math.sin(radians), Math.cos(radians),
Math.tan(radians));
// Display values for 60 degrees
degrees = 60;
radians = Math.toRadians(degrees);
System.out.printf("%-10d%-10.4f%-10.4f%-10.4f%-10.4f\n", degrees,
radians, Math.sin(radians), Math.cos(radians),
Math.tan(radians));
}
}
Degrees
30
60
Radians
0.5236
1.0472
Sine
0.5000
0.8660
Cosine
0.8660
0.5000
Tangent
0.5773
1.7320
The statement in lines 4–5 displays the column names of the table. The column names are
strings. Each string is displayed using the specifier %-10s, which left-justifies the string. The
statement in lines 10–12 displays the degrees as an integer and four float values. The integer is
displayed using the specifier %-10d and each float is displayed using the specifier %-10.4f,
which specifies four digits after the decimal point.
✓
Check
Point
4.22
4.23
What are the format specifiers for outputting a Boolean value, a character, a decimal
integer, a floating-point number, and a string?
What is wrong in the following statements?
(a) System.out.printf("%5d %d", 1, 2, 3);
(b) System.out.printf("%5d %f", 1);
(c) System.out.printf("%5d %f", 1, 2);
Chapter Summary 149
4.24
Show the output of the following statements.
(a) System.out.printf("amount is %f %e\n", 32.32, 32.32);
(b) System.out.printf("amount is %5.2%% %5.4e\n", 32.327, 32.32);
(c) System.out.printf("%6b\n", (1 > 2));
(d) System.out.printf("%6s\n", "Java");
(e) System.out.printf("%-6b%s\n", (1 > 2), "Java");
(f) System.out.printf("%6b%-8s\n", (1 > 2), "Java");
KEY TERMS
char type
125
encoding 125
escape character 127
escape sequence 126
format specifier 146
instance method 130
static method 130
supplementary Unicode 125
Unicode 125
whitespace character 133
CHAPTER SUMMARY
1. Java provides the mathematical methods sin, cos, tan, asin, acos,
atan, toRadians,
toDegree, exp, log, log10, pow, sqrt, cell, floor, rint, round, min, max,
abs, and random in the Math class for performing mathematical functions.
2. The character type char represents a single character.
3. An escape sequence consists of a backslash (\) followed by a character or a combination of digits.
4. The character \ is called the escape character.
5. The characters ' ', \t, \f, \r, and \n are known as the whitespace characters.
6. Characters can be compared based on their Unicode using the relational operators.
7. The Character class contains the methods isDigit, isLetter, isLetterOrDigit,
isLowerCase, isUpperCase for testing whether a character is a digit, letter, lowercase, and uppercase. It also contains the toLowerCase and toUpperCase methods for
returning a lowercase or uppercase letter.
8. A string is a sequence of characters. A string value is enclosed in matching double
quotes ("). A character value is enclosed in matching single quotes (').
9. Strings are objects in Java. A method that can only be invoked from a specific object is
called an instance method. A non-instance method is called a static method, which can
be invoked without using an object.
150 Chapter 4
Mathematical Functions, Characters, and Strings
10. You can get the length of a string by invoking its length() method, retrieve a character at the specified index in the string using the charAt(index) method, and
use the indexOf and lastIndexOf methods to find a character or a substring in
a string.
11. You can use the concat method to concatenate two strings, or the plus (+) operator to
concatenate two or more strings.
12. You can use the substring method to obtain a substring from the string.
13. You can use the equals and compareTo methods to compare strings. The equals
method returns true if two strings are equal, and false if they are not equal. The
compareTo method returns 0, a positive integer, or a negative integer, depending on
whether one string is equal to, greater than, or less than the other string.
14. The
printf method can be used to display a formatted output using format
specifiers.
QUIZ
Answer the quiz for this chapter online at www.cs.armstrong.edu/liang/intro10e/quiz.html.
PROGRAMMING EXERCISES
Section 4.2
4.1
(Geometry: area of a pentagon) Write a program that prompts the user to enter
the length from the center of a pentagon to a vertex and computes the area of the
pentagon, as shown in the following figure.
r
The formula for computing the area of a pentagon is Area =
5 * s2
, where
p
4 * tan¢ ≤
5
p
s is the length of a side. The side can be computed using the formula s = 2r sin ,
5
where r is the length from the center of a pentagon to a vertex. Round up two digits
after the decimal point. Here is a sample run:
Enter the length from the center to a vertex: 5.5
The area of the pentagon is 71.92
Programming Exercises 151
*4.2
(Geometry: great circle distance) The great circle distance is the distance between
two points on the surface of a sphere. Let (x1, y1) and (x2, y2) be the geographical latitude and longitude of two points. The great circle distance between the two
points can be computed using the following formula:
d = radius * arccos(sin(x1) * sin(x2) + cos(x1) * cos(x2) * cos(y1 - y2))
Write a program that prompts the user to enter the latitude and longitude of two
points on the earth in degrees and displays its great circle distance. The average
earth radius is 6,371.01 km. Note that you need to convert the degrees into radians
using the Math.toRadians method since the Java trigonometric methods use
radians. The latitude and longitude degrees in the formula are for north and west.
Use negative to indicate south and east degrees. Here is a sample run:
Enter point 1 (latitude and longitude) in degrees: 39.55, -116.25
Enter point 2 (latitude and longitude) in degrees: 41.5, 87.37
The distance between the two points is 10691.79183231593 km
*4.3
4.4
(Geography: estimate areas) Find the GPS locations for Atlanta, Georgia;
Orlando, Florida; Savannah, Georgia; and Charlotte, North Carolina from
www.gps-data-team.com/map/ and compute the estimated area enclosed by these
four cities. (Hint: Use the formula in Programming Exercise 4.2 to compute the
distance between two cities. Divide the polygon into two triangles and use the
formula in Programming Exercise 2.19 to compute the area of a triangle.)
(Geometry: area of a hexagon) The area of a hexagon can be computed using the
following formula (s is the length of a side):
Area =
6 * s2
p
4 * tan¢ ≤
6
Write a program that prompts the user to enter the side of a hexagon and displays
its area. Here is a sample run:
Enter the side: 5.5
The area of the hexagon is 78.59
*4.5
(Geometry: area of a regular polygon) A regular polygon is an n-sided polygon in
which all sides are of the same length and all angles have the same degree (i.e., the
polygon is both equilateral and equiangular). The formula for computing the area
of a regular polygon is
Area =
n * s2
p
4 * tan¢ ≤
n
VideoNote
Compute great circle distance
152 Chapter 4
Mathematical Functions, Characters, and Strings
Here, s is the length of a side. Write a program that prompts the user to enter the
number of sides and their length of a regular polygon and displays its area. Here is
a sample run:
Enter the number of sides: 5
Enter the side: 6.5
The area of the polygon is 74.69017017488385
*4.6
(Random points on a circle) Write a program that generates three random points
on a circle centered at (0, 0) with radius 40 and display three angles in a triangle
formed by these three points, as shown in Figure 4.7a. (Hint: Generate a random
angle a in radians between 0 and 2p, as shown in Figure 4.7b and the point determined by this angle is (r*cos(a), r*sin(a)).)
x = r × cos(α) and y = r ×sin(α)
0 o’clock position
p2
(x, y)
60
r
65
p3
p1
r
α
(0, 0)
55
p4
(a)
(b)
p5
(c)
FIGURE 4.7 (a) A triangle is formed from three random points on the circle. (b) A random
point on the circle can be generated using a random angle a. (c) A pentagon is centered at
(0, 0) with one point at the 0 o’clock position.
*4.7
(Corner point coordinates) Suppose a pentagon is centered at (0, 0) with one point
at the 0 o’clock position, as shown in Figure 4.7c. Write a program that prompts
the user to enter the radius of the bounding circle of a pentagon and displays the
coordinates of the five corner points on the pentagon. Here is a sample run:
Enter the radius of the bounding circle: 100
The coordinates of five points on the pentagon are
(95.1057, 30.9017)
(0.000132679, 100)
(-95.1056, 30.9019)
(-58.7788, -80.9015)
(58.7782, -80.902)
Sections 4.3–4.6
*4.8
(Find the character of an ASCII code) Write a program that receives an ASCII code
(an integer between 0 and 127) and displays its character. Here is a sample run:
Enter an ASCII code: 69
The character for ASCII code 69 is E
Programming Exercises 153
*4.9
(Find the Unicode of a character) Write a program that receives a character and
displays its Unicode. Here is a sample run:
Enter a character: E
The Unicode for the character E is 69
*4.10
(Guess birthday) Rewrite Listing 4.3, GuessBirthday.java, to prompt the user to
enter the character Y for Yes and N for No rather than entering 1 for Yes and 0
for No.
*4.11
(Decimal to hex) Write a program that prompts the user to enter an integer between
0 and 15 and displays its corresponding hex number. Here are some sample runs:
Enter a decimal value (0 to 15): 11
The hex value is B
Enter a decimal value (0 to 15): 5
The hex value is 5
Enter a decimal value (0 to 15): 31
31 is an invalid input
4.12
(Hex to binary) Write a program that prompts the user to enter a hex digit and
displays its corresponding binary number. Here is a sample run:
VideoNote
Convert hex to binary
Enter a hex digit: B
The binary value is 1011
Enter a hex digit: G
G is an invalid input
*4.13
(Vowel or consonant?) Write a program that prompts the user to enter a letter and
check whether the letter is a vowel or consonant. Here is a sample run:
Enter a letter: B
B is a consonant
Enter a letter grade: a
a is a vowel
Enter a letter grade: #
# is an invalid input
154 Chapter 4
Mathematical Functions, Characters, and Strings
*4.14
(Convert letter grade to number) Write a program that prompts the user to enter a
letter grade A, B, C, D, or F and displays its corresponding numeric value 4, 3, 2,
1, or 0. Here is a sample run:
Enter a letter grade: B
The numeric value for grade B is 3
Enter a letter grade: T
T is an invalid grade
*4.15
(Phone key pads) The international standard letter/number mapping found on the
telephone is shown below:
Write a program that prompts the user to enter a letter and displays its corresponding number.
Enter a letter: A
The corresponding number is 2
Enter a letter: a
The corresponding number is 2
Enter a letter: +
+ is an invalid input
4.16
*4.17
(Random character) Write a program that displays a random uppercase letter
using the Math.random() method.
(Days of a month) Write a program that prompts the user to enter a year and the
first three letters of a month name (with the first letter in uppercase) and displays
the number of days in the month. Here is a sample run:
Enter a year: 2001
Enter a month: Jan
Jan 2001 has 31 days
Programming Exercises 155
Enter a year: 2016
Enter a month: Feb
Jan 2016 has 29 days
*4.18
(Student major and status) Write a program that prompts the user to enter two
characters and displays the major and status represented in the characters. The first
character indicates the major and the second is number character 1, 2, 3, 4, which
indicates whether a student is a freshman, sophomore, junior, or senior. Suppose
the following chracters are used to denote the majors:
M: Mathematics
C: Computer Science
I: Information Technology
Here is a sample run:
Enter two characters: M1
Mathematics Freshman
Enter two characters: C3
Computer Science Junior
Enter two characters: T3
Invalid input
4.19
4.20
*4.21
(Business: check ISBN-10) Rewrite the Programming Exercise 3.9 by entering the
ISBN number as a string.
(Process a string) Write a program that prompts the user to enter a string and displays its length and its first character.
(Check SSN) Write a program that prompts the user to enter a Social Security
number in the format DDD-DD-DDDD, where D is a digit. Your program should
check whether the input is valid. Here are sample runs:
Enter a SSN: 232-23-5435
232-23-5435 is a valid social security number
Enter a SSN: 23-23-5435
23-23-5435 is an invalid social security number
4.22
(Check substring) Write a program that prompts the user to enter two strings and
reports whether the second string is a substring of the first string.
Enter string s1: ABCD
Enter string s2: BC
BC is a substring of ABCD
156 Chapter 4
Mathematical Functions, Characters, and Strings
Enter string s1: ABCD
Enter string s2: BDC
BDC is not a substring of ABCD
*4.23
(Financial application: payroll) Write a program that reads the following information and prints a payroll statement:
Employee’s name (e.g., Smith)
Number of hours worked in a week (e.g., 10)
Hourly pay rate (e.g., 9.75)
Federal tax withholding rate (e.g., 20%)
State tax withholding rate (e.g., 9%)
A sample run is shown below:
Enter
Enter
Enter
Enter
Enter
employee's name: Smith
number of hours worked in a week: 10
hourly pay rate: 9.75
federal tax withholding rate: 0.20
state tax withholding rate: 0.09
Employee Name: Smith
Hours Worked: 10.0
Pay Rate: $9.75
Gross Pay: $97.5
Deductions:
Federal Withholding (20.0%): $19.5
State Withholding (9.0%): $8.77
Total Deduction: $28.27
Net Pay: $69.22
*4.24
(Order three cities) Write a program that prompts the user to enter three cities and
displays them in ascending order. Here is a sample run:
Enter the
Enter the
Enter the
The three
*4.25
*4.26
first city: Chicago
second city: Los Angeles
third city: Atlanta
cities in alphabetical order are Atlanta Chicago Los Angeles
(Generate vehicle plate numbers) Assume a vehicle plate number consists of three
uppercase letters followed by four digits. Write a program to generate a plate
number.
(Financial application: monetary units) Rewrite Listing 2.10, ComputeChange.
java, to fix the possible loss of accuracy when converting a float value to an int
value. Read the input as a string such as "11.56". Your program should extract
the dollar amount before the decimal point and the cents after the decimal amount
using the indexOf and substring methods.
CHAPTER
5
LOOPS
Objectives
■
To write programs for executing statements repeatedly using a while
loop (§5.2).
■
To follow the loop design strategy to develop loops (§§5.2.1–5.2.3).
■
To control a loop with a sentinel value (§5.2.4).
■
To obtain large input from a file using input redirection rather than
typing from the keyboard (§5.2.5).
■
To write loops using do-while statements (§5.3).
■
To write loops using for statements (§5.4).
■
To discover the similarities and differences of three types of loop
statements (§5.5).
■
To write nested loops (§5.6).
■
To learn the techniques for minimizing numerical errors (§5.7).
■
To learn loops from a variety of examples (GCD, FutureTuition,
Dec2Hex) (§5.8).
■
To implement program control with break and continue (§5.9).
■
To process characters in a string using a loop in a case study for checking palindrome (§5.10).
■
To write a program that displays prime numbers (§5.11).
158 Chapter 5
Loops
5.1 Introduction
problem
Key
Point
A loop can be used to tell a program to execute statements repeatedly.
Suppose that you need to display a string (e.g., Welcome to Java!) a hundred times. It
would be tedious to have to write the following statement a hundred times:
100 times
System.out.println("Welcome to Java!");
System.out.println("Welcome to Java!");
...
System.out.println("Welcome to Java!");
So, how do you solve this problem?
Java provides a powerful construct called a loop that controls how many times an operation
or a sequence of operations is performed in succession. Using a loop statement, you simply
tell the computer to display a string a hundred times without having to code the print statement
a hundred times, as follows:
loop
int count = 0;
while (count < 100) {
System.out.println("Welcome to Java!");
count++;
}
The variable count is initially 0. The loop checks whether count < 100 is true. If so, it
executes the loop body to display the message Welcome to Java! and increments count
by 1. It repeatedly executes the loop body until count < 100 becomes false. When count
< 100 is false (i.e., when count reaches 100), the loop terminates and the next statement
after the loop statement is executed.
Loops are constructs that control repeated executions of a block of statements. The concept
of looping is fundamental to programming. Java provides three types of loop statements:
while loops, do-while loops, and for loops.
5.2 The while Loop
Key
Point
while loop
loop body
iteration
loop-continuationcondition
A while loop executes statements repeatedly while the condition is true.
The syntax for the while loop is:
while (loop-continuation-condition) {
// Loop body
Statement(s);
}
Figure 5.1a shows the while-loop flowchart. The part of the loop that contains the statements to be repeated is called the loop body. A one-time execution of a loop body is referred to
as an iteration (or repetition) of the loop. Each loop contains a loop-continuation-condition, a
Boolean expression that controls the execution of the body. It is evaluated each time to determine if the loop body is executed. If its evaluation is true, the loop body is executed; if its
evaluation is false, the entire loop terminates and the program control turns to the statement
that follows the while loop.
The loop for displaying Welcome to Java! a hundred times introduced in the preceding section is an example of a while loop. Its flowchart is shown in Figure 5.1b. The
5.2 The while Loop 159
count = 0;
loopcontinuationcondition?
false
(count < 100)?
true
false
true
Statement(s)
(loop body)
System.out.println("Welcome to Java!");
count++;
(a)
(b)
FIGURE 5.1
The while loop repeatedly executes the statements in the loop body when the
loop-continuation-condition evaluates to true.
loop-continuation-condition is count < 100 and the loop body contains the follow-
ing two statements:
loop-continuation-condition
int count = 0;
while (count < 100) {
System.out.printIn("Welcome to Java!");
count++;
}
loop body
In this example, you know exactly how many times the loop body needs to be executed
because the control variable count is used to count the number of executions. This type of
loop is known as a counter-controlled loop.
Note
The loop-continuation-condition must always appear inside the parentheses.
The braces enclosing the loop body can be omitted only if the loop body contains one
or no statement.
Here is another example to help understand how a loop works.
int sum = 0, i = 1;
while (i < 10) {
sum = sum + i;
i++;
}
System.out.println("sum is " + sum); // sum is 45
If i < 10 is true, the program adds i to sum. Variable i is initially set to 1, then is incremented to 2, 3, and up to 10. When i is 10, i < 10 is false, so the loop exits. Therefore,
the sum is 1 + 2 + 3 + ... + 9 = 45.
What happens if the loop is mistakenly written as follows?
int sum = 0, i = 1;
while (i < 10) {
sum = sum + i;
}
This loop is infinite, because i is always 1 and i < 10 will always be true.
counter-controlled loop
160 Chapter 5
Loops
Note
Make sure that the loop-continuation-condition eventually becomes false
so that the loop will terminate. A common programming error involves infinite loops
(i. e., the loop runs forever). If your program takes an unusually long time to run and
does not stop, it may have an infinite loop. If you are running the program from the
command window, press CTRL+C to stop it.
infinite loop
Caution
Programmers often make the mistake of executing a loop one more or less time. This
is commonly known as the off-by-one error. For example, the following loop displays
Welcome to Java 101 times rather than 100 times. The error lies in the condition,
which should be count < 100 rather than count <= 100.
off-by-one error
int count = 0;
while (count <= 100) {
System.out.println("Welcome to Java!");
count++;
}
Recall that Listing 3.1, AdditionQuiz.java, gives a program that prompts the user to enter
an answer for a question on addition of two single digits. Using a loop, you can now rewrite
the program to let the user repeatedly enter a new answer until it is correct, as shown in
Listing 5.1.
LISTING 5.1 RepeatAdditionQuiz.java
generate number1
generate number2
show question
get first answer
check answer
read an answer
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
import java.util.Scanner;
public class RepeatAdditionQuiz {
public static void main(String[] args) {
int number1 = (int)(Math.random() * 10);
int number2 = (int)(Math.random() * 10);
// Create a Scanner
Scanner input = new Scanner(System.in);
System.out.print(
"What is " + number1 + " + " + number2 + "? ");
int answer = input.nextInt();
while (number1 + number2 != answer) {
System.out.print("Wrong answer. Try again. What is "
+ number1 + " + " + number2 + "? ");
answer = input.nextInt();
}
System.out.println("You got it!");
}
}
What is 5 + 9? 12
Wrong answer. Try again. What is 5 + 9? 34
Wrong answer. Try again. What is 5 + 9? 14
You got it!
5.2 The while Loop 161
The loop in lines 15–19 repeatedly prompts the user to enter an answer when number1
+ number2 != answer is true. Once number1 + number2 != answer is false, the
loop exits.
5.2.1
Case Study: Guessing Numbers
The problem is to guess what number a computer has in mind. You will write a program that
randomly generates an integer between 0 and 100, inclusive. The program prompts the user
to enter a number continuously until the number matches the randomly generated number. For
each user input, the program tells the user whether the input is too low or too high, so the user
can make the next guess intelligently. Here is a sample run:
VideoNote
Guess a number
Guess a magic number between 0 and 100
Enter your guess: 50
Your guess is too high
Enter your guess: 25
Your guess is too low
Enter your guess: 42
Your guess is too high
Enter your guess: 39
Yes, the number is 39
The magic number is between 0 and 100. To minimize the number of guesses, enter 50
first. If your guess is too high, the magic number is between 0 and 49. If your guess is too
low, the magic number is between 51 and 100. So, you can eliminate half of the numbers from
further consideration after one guess.
How do you write this program? Do you immediately begin coding? No. It is important to
think before coding. Think how you would solve the problem without writing a program. You
need first to generate a random number between 0 and 100, inclusive, then to prompt the user
to enter a guess, and then to compare the guess with the random number.
It is a good practice to code incrementally one step at a time. For programs involving loops,
if you don’t know how to write a loop right away, you may first write the code for executing
the loop one time, and then figure out how to repeatedly execute the code in a loop. For this
program, you may create an initial draft, as shown in Listing 5.2.
intelligent guess
think before coding
code incrementally
LISTING 5.2 GuessNumberOneTime.java
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
import java.util.Scanner;
public class GuessNumberOneTime {
public static void main(String[] args) {
// Generate a random number to be guessed
int number = (int)(Math.random() * 101);
generate a number
Scanner input = new Scanner(System.in);
System.out.println("Guess a magic number between 0 and 100");
// Prompt the user to guess the number
System.out.print("\nEnter your guess: ");
int guess = input.nextInt();
enter a guess
if (guess == number)
System.out.println("Yes, the number is " + number);
correct guess?
162 Chapter 5
too high?
too low?
Loops
17
18
19
20
21
22
else if (guess > number)
System.out.println("Your guess is too high");
else
System.out.println("Your guess is too low");
}
}
When you run this program, it prompts the user to enter a guess only once. To let the user
enter a guess repeatedly, you may wrap the code in lines 11–20 in a loop as follows:
while (true) {
// Prompt the user to guess the number
System.out.print("\nEnter your guess: ");
guess = input.nextInt();
if (guess == number)
System.out.println("Yes, the number is " + number);
else if (guess > number)
System.out.println("Your guess is too high");
else
System.out.println("Your guess is too low");
} // End of loop
This loop repeatedly prompts the user to enter a guess. However, this loop is not correct,
because it never terminates. When guess matches number, the loop should end. So, the loop
can be revised as follows:
while (guess != number) {
// Prompt the user to guess the number
System.out.print("\nEnter your guess: ");
guess = input.nextInt();
if (guess == number)
System.out.println("Yes, the number is " + number);
else if (guess > number)
System.out.println("Your guess is too high");
else
System.out.println("Your guess is too low");
} // End of loop
The complete code is given in Listing 5.3.
LISTING 5.3 GuessNumber.java
generate a number
1
2
3
4
5
6
7
8
9
10
11
12
13
14
import java.util.Scanner;
public class GuessNumber {
public static void main(String[] args) {
// Generate a random number to be guessed
int number = (int)(Math.random() * 101);
Scanner input = new Scanner(System.in);
System.out.println("Guess a magic number between 0 and 100");
int guess = -1;
while (guess != number) {
// Prompt the user to guess the number
System.out.print("\nEnter your guess: ");
5.2 The while Loop 163
15
16
17
18
19
20
21
22
23
24
25
guess = input.nextInt();
enter a guess
if (guess == number)
System.out.println("Yes, the number is " + number);
else if (guess > number)
System.out.println("Your guess is too high");
else
System.out.println("Your guess is too low");
} // End of loop
}
}
line#
6
iteration 1
iteration 2
iteration 3
iteration 4
number
guess
output
39
11
–1
15
50
20
15
Your guess is too high
25
22
15
Your guess is too low
42
20
15
Your guess is too high
39
18
Yes, the number is 39
The program generates the magic number in line 6 and prompts the user to enter a guess
continuously in a loop (lines 12–23). For each guess, the program checks whether the guess
is correct, too high, or too low (lines 17–22). When the guess is correct, the program exits the
loop (line 12). Note that guess is initialized to -1. Initializing it to a value between 0 and 100
would be wrong, because that could be the number to be guessed.
5.2.2
Loop Design Strategies
Writing a correct loop is not an easy task for novice programmers. Consider three steps when
writing a loop.
Step 1: Identify the statements that need to be repeated.
Step 2: Wrap these statements in a loop like this:
while (true) {
Statements;
}
Step 3: Code the loop-continuation-condition and add appropriate statements for
controlling the loop.
while (loop-continuation-condition) {
Statements;
Additional statements for controlling the loop;
}
too high?
too low?
164 Chapter 5
Loops
5.2.3
VideoNote
Multiple subtraction quiz
Case Study: Multiple Subtraction Quiz
The Math subtraction learning tool program in Listing 3.3, SubtractionQuiz.java, generates just
one question for each run. You can use a loop to generate questions repeatedly. How do you
write the code to generate five questions? Follow the loop design strategy. First identify the
statements that need to be repeated. These are the statements for obtaining two random numbers,
prompting the user with a subtraction question, and grading the question. Second, wrap the statements in a loop. Third, add a loop control variable and the loop-continuation-condition
to execute the loop five times.
Listing 5.4 gives a program that generates five questions and, after a student answers all
five, reports the number of correct answers. The program also displays the time spent on the
test and lists all the questions.
LISTING 5.4 SubtractionQuizLoop.java
get start time
loop
display a question
grade an answer
increase correct count
increase control variable
prepare output
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
import java.util.Scanner;
public class SubtractionQuizLoop {
public static void main(String[] args) {
final int NUMBER_OF_QUESTIONS = 5; // Number of questions
int correctCount = 0; // Count the number of correct answers
int count = 0; // Count the number of questions
long startTime = System.currentTimeMillis();
String output = " "; // output string is initially empty
Scanner input = new Scanner(System.in);
while (count < NUMBER_OF_QUESTIONS) {
// 1. Generate two random single-digit integers
int number1 = (int)(Math.random() * 10);
int number2 = (int)(Math.random() * 10);
// 2. If number1 < number2, swap number1 with number2
if (number1 < number2) {
int temp = number1;
number1 = number2;
number2 = temp;
}
// 3. Prompt the student to answer "What is number1 – number2?"
System.out.print(
"What is " + number1 + " - " + number2 + "? ");
int answer = input.nextInt();
// 4. Grade the answer and display the result
if (number1 - number2 == answer) {
System.out.println("You are correct!");
correctCount++; // Increase the correct answer count
}
else
System.out.println("Your answer is wrong.\n" + number1
+ " - " + number2 + " should be " + (number1 - number2));
// Increase the question count
count++;
output += "\n" + number1 + "-" + number2 + "=" + answer +
((number1 - number2 == answer) ? " correct" : " wrong");
5.2 The while Loop 165
43
44
45
46
47
48
49
50
51
}
end loop
long endTime = System.currentTimeMillis();
long testTime = endTime - startTime;
get end time
test time
System.out.println("Correct count is " + correctCount +
"\nTest time is " + testTime / 1000 + " seconds\n" + output);
display result
}
}
What is 9 – 2? 7
You are correct!
What is 3 – 0? 3
You are correct!
What is 3 – 2? 1
You are correct!
What is 7 – 4? 4
Your answer is wrong.
7 – 4 should be 3
What is 7 – 5? 4
Your answer is wrong.
7 – 5 should be 2
Correct count is 3
Test time is 1021 seconds
9–2=7
3–0=3
3–2=1
7–4=4
7–5=4
correct
correct
correct
wrong
wrong
The program uses the control variable count to control the execution of the loop. count
is initially 0 (line 7) and is increased by 1 in each iteration (line 39). A subtraction question is
displayed and processed in each iteration. The program obtains the time before the test starts
in line 8 and the time after the test ends in line 45, and computes the test time in line 46. The
test time is in milliseconds and is converted to seconds in line 49.
5.2.4
Controlling a Loop with a Sentinel Value
Another common technique for controlling a loop is to designate a special value when reading and processing a set of values. This special input value, known as a sentinel value, signifies the end of the input. A loop that uses a sentinel value to control its execution is called a
sentinel-controlled loop.
Listing 5.5 writes a program that reads and calculates the sum of an unspecified number
of integers. The input 0 signifies the end of the input. Do you need to declare a new variable
for each input value? No. Just use one variable named data (line 12) to store the input value
and use a variable named sum (line 15) to store the total. Whenever a value is read, assign it
to data and, if it is not zero, add it to sum (line 17).
sentinel value
sentinel-controlled loop
166 Chapter 5
Loops
LISTING 5.5 SentinelValue.java
input
loop
end of loop
display result
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
import java.util.Scanner;
public class SentinelValue {
/** Main method */
public static void main(String[] args) {
// Create a Scanner
Scanner input = new Scanner(System.in);
// Read an initial data
System.out.print(
"Enter an integer (the input ends if it is 0): ");
int data = input.nextInt();
// Keep reading data until the input is 0
int sum = 0;
while (data != 0) {
sum += data;
// Read the next data
System.out.print(
"Enter an integer (the input ends if it is 0): ");
data = input.nextInt();
}
System.out.println("The sum is " + sum);
}
}
Enter an integer (the input ends if it is 0): 2
Enter an integer (the input ends if it is 0): 3
Enter an integer (the input ends if it is 0): 4
Enter an integer (the input ends if it is 0): 0
The sum is 9
line#
12
iteration 1
iteration 2
iteration 3
Data
sum
2
15
0
17
2
22
3
17
22
5
4
17
22
25
output
9
0
The sum is 9
If data is not 0, it is added to sum (line 17) and the next item of input data is read (lines
20–22). If data is 0, the loop body is no longer executed and the while loop terminates. The
input value 0 is the sentinel value for this loop. Note that if the first input read is 0, the loop
body never executes, and the resulting sum is 0.
5.2 The while Loop 167
Caution
Don’t use floating-point values for equality checking in a loop control. Because floatingpoint values are approximations for some values, using them could result in imprecise
counter values and inaccurate results.
Consider the following code for computing 1 + 0.9 + 0.8 + ... + 0.1:
double item = 1; double sum = 0;
while (item != 0) { // No guarantee item will be 0
sum += item;
item -= 0.1;
}
System.out.println(sum);
Variable item starts with 1 and is reduced by 0.1 every time the loop body is executed.
The loop should terminate when item becomes 0. However, there is no guarantee that
item will be exactly 0, because the floating-point arithmetic is approximated. This loop
seems okay on the surface, but it is actually an infinite loop.
5.2.5
numeric error
Input and Output Redirections
In the preceding example, if you have a large number of data to enter, it would be cumbersome
to type from the keyboard. You can store the data separated by whitespaces in a text file, say
input.txt, and run the program using the following command:
java SentinelValue < input.txt
This command is called input redirection. The program takes the input from the file input
.txt rather than having the user type the data from the keyboard at runtime. Suppose the contents of the file are
input redirection
2 3 4 5 6 7 8 9 12 23 32
23 45 67 89 92 12 34 35 3 1 2 4 0
The program should get sum to be 518.
Similarly, there is output redirection, which sends the output to a file rather than displaying
it on the console. The command for output redirection is:
output redirection
java ClassName > output.txt
Input and output redirection can be used in the same command. For example, the following
command gets input from input.txt and sends output to output.txt:
java SentinelValue
output.txt
Try running the program to see what contents are in output.txt.
5.1
Analyze the following code. Is count < 100 always true, always false, or sometimes true or sometimes false at Point A, Point B, and Point C?
int count = 0;
while (count < 100) {
// Point A
System.out.println("Welcome to Java!");
count++;
// Point B
}
// Point C
✓
Check
Point
168 Chapter 5
Loops
5.2
5.3
What is wrong if guess is initialized to 0 in line 11 in Listing 5.3?
How many times are the following loop bodies repeated? What is the output of each
loop?
int i = 1;
while (i < 10)
if (i % 2 == 0)
System.out.println(i);
int i = 1;
while (i < 10)
if (i % 2 == 0)
System.out.println(i++);
(a)
int i = 1;
while (i < 10)
if ((i++) % 2 == 0)
System.out.println(i);
(b)
5.4
(c)
Suppose the input is 2 3 4 5 0. What is the output of the following code?
import java.util.Scanner;
public class Test {
public static void main(String[] args) {
Scanner input = new Scanner(System.in);
int number, max;
number = input.nextInt();
max = number;
while (number != 0) {
number = input.nextInt();
if (number > max)
max = number;
}
System.out.println("max is " + max);
System.out.println("number " + number);
}
}
5.5
What is the output of the following code? Explain the reason.
int x = 80000000;
while (x > 0)
x++;
System.out.println("x is " + x);
5.3 The do-while Loop
Key
Point
A do-while loop is the same as a while loop except that it executes the loop body
first and then checks the loop continuation condition.
The do-while loop is a variation of the while loop. Its syntax is:
do-while loop
do {
// Loop body;
Statement(s);
} while (loop-continuation-condition);
Its execution flowchart is shown in Figure 5.2.
The loop body is executed first, and then the loop-continuation-condition is evaluated. If the evaluation is true, the loop body is executed again; if it is false, the do-while
5.3 The do-while Loop 169
Statement(s)
(loop body)
true
loopcontinuationcondition?
false
FIGURE 5.2 The do-while loop executes the loop body first, then checks the loopcontinuation-condition to determine whether to continue or terminate the loop.
loop terminates. The difference between a while loop and a do-while loop is the order in
which the loop-continuation-condition is evaluated and the loop body executed. You
can write a loop using either the while loop or the do-while loop. Sometimes one is a more
convenient choice than the other. For example, you can rewrite the while loop in Listing 5.5
using a do-while loop, as shown in Listing 5.6.
LISTING 5.6 TestDoWhile.java
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
import java.util.Scanner;
public class TestDoWhile {
/** Main method */
public static void main(String[] args) {
int data;
int sum = 0;
// Create a Scanner
Scanner input = new Scanner(System.in);
// Keep reading data until the input is 0
do {
// Read the next data
System.out.print(
"Enter an integer (the input ends if it is 0): ");
data = input.nextInt();
sum += data;
} while (data != 0);
System.out.println("The sum is " + sum);
}
}
Enter an integer (the input ends if it is 0): 3
Enter an integer (the input ends if it is 0): 5
Enter an integer (the input ends if it is 0): 6
Enter an integer (the input ends if it is 0): 0
The sum is 14
loop
end loop
170 Chapter 5
Loops
Tip
Use a do-while loop if you have statements inside the loop that must be executed
at least once, as in the case of the do-while loop in the preceding TestDoWhile
program. These statements must appear before the loop as well as inside it if you use
a while loop.
✓
Check
Point
5.6
Suppose the input is 2 3 4 5 0. What is the output of the following code?
import java.util.Scanner;
public class Test {
public static void main(String[] args) {
Scanner input = new Scanner(System.in);
int number, max;
number = input.nextInt();
max = number;
do {
number = input.nextInt();
if (number > max)
max = number;
} while (number != 0);
System.out.println("max is " + max);
System.out.println("number " + number);
}
}
5.7
What are the differences between a while loop and a do-while loop? Convert the
following while loop into a do-while loop.
Scanner input = new Scanner(System.in);
int sum = 0;
System.out.println("Enter an integer " +
"(the input ends if it is 0)");
int number = input.nextInt();
while (number != 0) {
sum += number;
System.out.println("Enter an integer " +
"(the input ends if it is 0)");
number = input.nextInt();
}
5.4 The for Loop
Key
Point
A for loop has a concise syntax for writing loops.
Often you write a loop in the following common form:
i = initialValue; // Initialize loop control variable
while (i < endValue)
// Loop body
...
i++; // Adjust loop control variable
}
5.4 The for Loop 171
A for loop can be used to simplify the preceding loop as:
for (i = initialValue; i < endValue; i++)
// Loop body
...
}
In general, the syntax of a for loop is:
for (initial-action; loop-continuation-condition;
action-after-each-iteration) {
// Loop body;
Statement(s);
}
for loop
The flowchart of the for loop is shown in Figure 5.3a.
i = 0
initial-action
loopcontinuationcondition?
false
true
(i < 100)?
false
true
Statement(s)
(loop body)
System.out.println(
"Welcome to Java");
action-after-each-iteration
i++
(a)
(b)
FIGURE 5.3 A for loop performs an initial action once, then repeatedly executes
the statements in the loop body, and performs an action after an iteration when the
loop-continuation-condition evaluates to true.
The for loop statement starts with the keyword for, followed by a pair of parentheses enclosing the control structure of the loop. This structure consists of initial-action,
loop-continuation-condition, and action-after-each-iteration. The control
structure is followed by the loop body enclosed inside braces. The initial-action, loopcontinuation-condition, and action-after-each-iteration are separated by
semicolons.
A for loop generally uses a variable to control how many times the loop body is executed
and when the loop terminates. This variable is referred to as a control variable. The initialaction often initializes a control variable, the action-after-each-iteration usually
increments or decrements the control variable, and the loop-continuation-condition
control variable
172 Chapter 5
Loops
tests whether the control variable has reached a termination value. For example, the following
for loop prints Welcome to Java! a hundred times:
int i;
for (i = 0; i < 100; i++) {
System.out.println("Welcome to Java!");
}
initial-action
action-after-each-iteration
The flowchart of the statement is shown in Figure 5.3b. The for loop initializes i to 0, then
repeatedly executes the println statement and evaluates i++ while i is less than 100.
The initial-action, i = 0, initializes the control variable, i. The loopcontinuation-condition, i < 100, is a Boolean expression. The expression is evaluated
right after the initialization and at the beginning of each iteration. If this condition is true,
the loop body is executed. If it is false, the loop terminates and the program control turns to
the line following the loop.
The action-after-each-iteration, i++, is a statement that adjusts the control variable.
This statement is executed after each iteration and increments the control variable. Eventually,
the value of the control variable should force the loop-continuation-condition to
become false; otherwise, the loop is infinite.
The loop control variable can be declared and initialized in the for loop. Here is an example:
for (int i = 0; i < 100; i++) {
System.out.println("Welcome to Java!");
}
omitting braces
If there is only one statement in the loop body, as in this example, the braces can be omitted.
Tip
declare control variable
The control variable must be declared inside the control structure of the loop or before
the loop. If the loop control variable is used only in the loop, and not elsewhere, it is a
good programming practice to declare it in the initial-action of the for loop. If
the variable is declared inside the loop control structure, it cannot be referenced outside
the loop. In the preceding code, for example, you cannot reference i outside the for
loop, because it is declared inside the for loop.
Note
for loop variations
The initial-action in a for loop can be a list of zero or more comma-separated
variable declaration statements or assignment expressions. For example:
for (int i = 0, j = 0; i + j < 10; i++, j++) {
// Do something
}
The action-after-each-iteration in a for loop can be a list of zero or more
comma-separated statements. For example:
for (int i = 1; i < 100; System.out.println(i), i++);
This example is correct, but it is a bad example, because it makes the code difficult to
read. Normally, you declare and initialize a control variable as an initial action and increment or decrement the control variable as an action after each iteration.
Note
If the loop-continuation-condition in a for loop is omitted, it is implicitly
true. Thus the statement given below in (a), which is an infinite loop, is the same as
in (b). To avoid confusion, though, it is better to use the equivalent loop in (c).
5.4 The for Loop 173
for ( ; ; ) {
// Do something
}
Equivalent
for ( ; true; ) {
// Do something
}
(a)
5.8
(b)
Equivalent
This is better
for (int i = 0; i < 10; i++) {
sum += i;
}
(b)
(a)
5.10
(c)
Do the following two loops result in the same value in sum?
for (int i = 0; i < 10; ++i) {
sum += i;
}
5.9
while (true) {
// Do something
}
What are the three parts of a for loop control? Write a for loop that prints the numbers from 1 to 100.
Suppose the input is 2 3 4 5 0. What is the output of the following code?
import java.util.Scanner;
public class Test {
public static void main(String[] args) {
Scanner input = new Scanner(System.in);
int number, sum = 0, count;
for (count = 0; count < 5; count++) {
number = input.nextInt();
sum += number;
}
System.out.println("sum is " + sum);
System.out.println("count is " + count);
}
}
5.11
What does the following statement do?
for ( ; ; ) {
// Do something
}
5.12
5.13
If a variable is declared in a for loop control, can it be used after the loop exits?
Convert the following for loop statement to a while loop and to a do-while loop:
long sum = 0;
for (int i = 0; i <= 1000; i++)
sum = sum + i;
5.14
Count the number of iterations in the following loops.
int count = 0;
while (count < n) {
count++;
}
(a)
for (int count = 0;
count <= n; count++) {
}
(b)
✓
Check
Point
174 Chapter 5
Loops
int count = 5;
while (count < n) {
count++;
}
int count = 5;
while (count < n) {
count = count + 3;
}
(d)
(c)
5.5 Which Loop to Use?
pretest loop
posttest loop
Key
Point
You can use a for loop, a while loop, or a do-while loop, whichever is convenient.
The while loop and for loop are called pretest loops because the continuation condition
is checked before the loop body is executed. The do-while loop is called a posttest loop
because the condition is checked after the loop body is executed. The three forms of loop
statements—while, do-while, and for—are expressively equivalent; that is, you can write
a loop in any of these three forms. For example, a while loop in (a) in the following figure
can always be converted into the for loop in (b).
while (loop-continuation-condition) {
// Loop body
}
for ( ; loop-continuation-condition; ) {
// Loop body
}
Equivalent
(a)
(b)
A for loop in (a) in the next figure can generally be converted into the while loop in
(b) except in certain special cases (see Checkpoint Question 5.25 for such a case).
for (initial-action;
loop-continuation-condition;
action-after-each-iteration) {
// Loop body;
}
Equivalent
initial-action;
while (loop-continuation-condition) {
// Loop body;
action-after-each-iteration;
}
(b)
(a)
Use the loop statement that is most intuitive and comfortable for you. In general, a for
loop may be used if the number of repetitions is known in advance, as, for example, when
you need to display a message a hundred times. A while loop may be used if the number of
repetitions is not fixed, as in the case of reading the numbers until the input is 0. A do-while
loop can be used to replace a while loop if the loop body has to be executed before the continuation condition is tested.
Caution
Adding a semicolon at the end of the for clause before the loop body is a common
mistake, as shown below in (a). In (a), the semicolon signifies the end of the loop prematurely. The loop body is actually empty, as shown in (b). (a) and (b) are equivalent.
Both are incorrect.
Empty body
Error
for (int i = 0; i < 10; i++);
{
System.out.println("i is " + i);
}
for (int i = 0; i < 10; i++) { };
{
System.out.println("i is " + i);
}
(a)
(b)
5.5 Which Loop to Use? 175
Similarly, the loop in (c) is also wrong. (c) is equivalent to (d). Both are incorrect.
Empty body
Error
int i = 0;
while (i < 10);
{
System.out.println("i is " + i);
i++;
}
int i = 0;
while (i < 10) { };
{
System.out.println("i is " + i);
i++;
}
(c)
(d)
These errors often occur when you use the next-line block style. Using the end-of-line
block style can avoid errors of this type.
In the case of the do-while loop, the semicolon is needed to end the loop.
int i = 0;
do {
System.out.println("i is " + i);
i++;
} while (i < 10);
Correct
5.15
5.16
Can you convert a for loop to a while loop? List the advantages of using for loops.
Can you always convert a while loop into a for loop? Convert the following while
loop into a for loop.
int i = 1;
int sum = 0;
while (sum < 10000) {
sum = sum + i;
i++;
}
5.17
Identify and fix the errors in the following code:
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
public class Test {
public void main(String[] args) {
for (int i = 0; i < 10; i++);
sum += i;
if (i < j);
System.out.println(i)
else
System.out.println(j);
while (j < 10);
{
j++;
}
do {
j++;
} while (j < 10)
}
}
✓
Check
Point
176 Chapter 5
Loops
5.18
What is wrong with the following programs?
1 public class ShowErrors {
2
public static void main(String[] args) {
3
int i = 0;
4
do {
5
System.out.println(i + 4);
6
i++;
7
}
8
while (i < 10)
9
}
10 }
1 public class ShowErrors {
2
public static void main(String[] args) {
3
for (int i = 0; i < 10; i++);
4
System.out.println(i + 4);
5
}
6 }
(a)
(b)
5.6 Nested Loops
nested loop
Key
Point
A loop can be nested inside another loop.
Nested loops consist of an outer loop and one or more inner loops. Each time the outer loop is
repeated, the inner loops are reentered, and started anew.
Listing 5.7 presents a program that uses nested for loops to display a multiplication table.
LISTING 5.7 MultiplicationTable.java
table title
outer loop
inner loop
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
public class MultiplicationTable {
/** Main method */
public static void main(String[] args) {
// Display the table heading
System.out.println("
Multiplication Table");
// Display the number title
System.out.print("
");
for (int j = 1; j <= 9; j++)
System.out.print("
" + j);
System.out.println("\n———————————————————————————————————————");
// Display table body
for (int i = 1; i <= 9; i++) {
System.out.print(i + " | ");
for (int j = 1; j <= 9; j++) {
// Display the product and align properly
System.out.printf("%4d", i * j);
}
System.out.println();
}
}
}
Multiplication Table
1
2
3
4
5
6
7
8
9
———————————————————————————————————————1 |
1
2
3
4
5
6
7
8
9
2 |
2
4
6
8 10 12 14 16 18
3 |
3
6
9 12 15 18 21 24 27
4 |
4
8 12 16 20 24 28 32 36
5 |
5 10 15 20 25 30 35 40 45
6 |
6 12 18 24 30 36 42 48 54
7 |
7 14 21 28 35 42 49 56 63
8 |
8 16 24 32 40 48 56 64 72
9 |
9 18 27 36 45 54 63 72 81
5.6 Nested Loops 177
The program displays a title (line 5) on the first line in the output. The first for loop (lines
9–10) displays the numbers 1 through 9 on the second line. A dashed (-) line is displayed on
the third line (line 12).
The next loop (lines 15–22) is a nested for loop with the control variable i in the outer
loop and j in the inner loop. For each i, the product i * j is displayed on a line in the inner
loop, with j being 1, 2, 3, . . ., 9.
Note
Be aware that a nested loop may take a long time to run. Consider the following loop
nested in three levels:
for (int i = 0; i < 10000; i++)
for (int j = 0; j < 10000; j++)
for (int k = 0; k < 10000; k++)
Perform an action
The action is performed one trillion times. If it takes 1 microsecond to perform the action,
the total time to run the loop would be more than 277 hours. Note that 1 microsecond
is one millionth (10– 6) of a second.
5.19
✓
How many times is the println statement executed?
Check
Point
for (int i = 0; i < 10; i++)
for (int j = 0; j < i; j++)
System.out.println(i * j)
5.20
Show the output of the following programs. (Hint: Draw a table and list the variables
in the columns to trace these programs.)
public class Test {
public static void main(String[] args) {
for (int i = 1; i < 5; i++) {
int j = 0;
while (j < i) {
System.out.print(j + " ");
j++;
}
}
}
}
public class Test {
public static void main(String[] args) {
int i = 0;
while (i < 5) {
for (int j = i; j > 1; j--)
System.out.print(j + " ");
System.out.println("****");
i++;
}
}
}
(a)
public class Test {
public static void main(String[] args) {
int i = 5;
while (i >= 1) {
int num = 1;
for (int j = 1; j <= i; j++) {
System.out.print(num + "xxx");
num *= 2;
}
(b)
public class Test {
public static void main(String[] args) {
int i = 1;
do {
int num = 1;
for (int j = 1; j <= i; j++) {
System.out.print(num + "G");
num += 2;
}
System.out.println();
i--;
System.out.println();
i++;
} while (i <= 5);
}
}
}
}
}
(c)
(d)
178 Chapter 5
Loops
5.7 Minimizing Numeric Errors
Key
Point
VideoNote
Minimize numeric errors
Using floating-point numbers in the loop continuation condition may cause numeric errors.
Numeric errors involving floating-point numbers are inevitable, because floating-point numbers are represented in approximation in computers by nature. This section discusses how to
minimize such errors through an example.
Listing 5.8 presents an example summing a series that starts with 0.01 and ends with 1.0.
The numbers in the series will increment by 0.01, as follows: 0.01 + 0.02 + 0.03, and so on.
LISTING 5.8 TestSum.java
loop
1
2
3
4
5
6
7
8
9
10
11
12
13
public class TestSum {
public static void main(String[] args) {
// Initialize sum
float sum = 0;
// Add 0.01, 0.02, ..., 0.99, 1 to sum
for (float i = 0.01f; i <= 1.0f; i = i + 0.01f)
sum += i;
// Display result
System.out.println("The sum is " + sum);
}
}
The sum is 50.499985
double precision
The for loop (lines 7–8) repeatedly adds the control variable i to sum. This variable, which
begins with 0.01, is incremented by 0.01 after each iteration. The loop terminates when i
exceeds 1.0.
The for loop initial action can be any statement, but it is often used to initialize a control
variable. From this example, you can see that a control variable can be a float type. In fact,
it can be any data type.
The exact sum should be 50.50, but the answer is 50.499985. The result is imprecise
because computers use a fixed number of bits to represent floating-point numbers, and thus
they cannot represent some floating-point numbers exactly. If you change float in the program to double, as follows, you should see a slight improvement in precision, because a
double variable holds 64 bits, whereas a float variable holds 32 bits.
// Initialize sum
double sum = 0;
// Add 0.01, 0.02, ..., 0.99, 1 to sum
for (double i = 0.01; i <= 1.0; i = i + 0.01)
sum += i;
numeric error
However, you will be stunned to see that the result is actually 49.50000000000003. What
went wrong? If you display i for each iteration in the loop, you will see that the last i is
slightly larger than 1 (not exactly 1). This causes the last i not to be added into sum. The
fundamental problem is that the floating-point numbers are represented by approximation. To
fix the problem, use an integer count to ensure that all the numbers are added to sum. Here is
the new loop:
double currentValue = 0.01;
for (int count = 0; count < 100; count++) {
5.8 Case Studies 179
sum += currentValue;
currentValue += 0.01;
}
After this loop, sum is 50.50000000000003. This loop adds the numbers from smallest to
biggest. What happens if you add numbers from biggest to smallest (i.e., 1.0, 0.99, 0.98,
. . . , 0.02, 0.01 in this order) as follows:
double currentValue = 1.0;
for (int count = 0; count < 100; count++) {
sum += currentValue;
currentValue -= 0.01;
}
After this loop, sum is 50.49999999999995. Adding from biggest to smallest is less accurate
than adding from smallest to biggest. This phenomenon is an artifact of the finite-precision arithmetic. Adding a very small number to a very big number can have no effect if the result requires
more precision than the variable can store. For example, the inaccurate result of 100000000.0
+ 0.000000001 is 100000000.0. To obtain more accurate results, carefully select the order
of computation. Adding smaller numbers before bigger numbers is one way to minimize errors.
avoiding numeric error
5.8 Case Studies
Loops are fundamental in programming. The ability to write loops is essential in
learning Java programming.
Key
Point
If you can write programs using loops, you know how to program! For this reason, this section
presents four additional examples of solving problems using loops.
5.8.1
Case Study: Finding the Greatest Common Divisor
The greatest common divisor (gcd) of the two integers 4 and 2 is 2. The greatest common
divisor of the two integers 16 and 24 is 8. How would you write this program to find the greatest common divisor? Would you immediately begin to write the code? No. It is important to
think before you code. Thinking enables you to generate a logical solution for the problem
without concern about how to write the code.
Let the two input integers be n1 and n2. You know that number 1 is a common divisor, but
it may not be the greatest common divisor. So, you can check whether k (for k = 2, 3, 4, and
so on) is a common divisor for n1 and n2, until k is greater than n1 or n2. Store the common
divisor in a variable named gcd. Initially, gcd is 1. Whenever a new common divisor is found,
it becomes the new gcd. When you have checked all the possible common divisors from 2 up
to n1 or n2, the value in variable gcd is the greatest common divisor. Once you have a logical
solution, type the code to translate the solution into a Java program as follows:
int gcd = 1; // Initial gcd is 1
int k = 2; // Possible gcd
while (k <= n1 && k <= n2) {
if (n1 % k == 0 && n2 % k == 0)
gcd = k; // Update gcd
k++; // Next possible gcd
}
// After the loop, gcd is the greatest common divisor for n1 and n2
Listing 5.9 presents the program that prompts the user to enter two positive integers and
finds their greatest common divisor.
gcd
think before you code
logical solution
180 Chapter 5
Loops
LISTING 5.9 GreatestCommonDivisor.java
input
input
gcd
check divisor
output
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
import java.util.Scanner;
public class GreatestCommonDivisor {
/** Main method */
public static void main(String[] args) {
// Create a Scanner
Scanner input = new Scanner(System.in);
// Prompt the user to enter two integers
System.out.print("Enter first integer: ");
int n1 = input.nextInt();
System.out.print("Enter second integer: ");
int n2 = input.nextInt();
int gcd = 1; // Initial gcd is 1
int k = 2; // Possible gcd
while (k <= n1 && k <= n2) {
if (n1 % k == 0 && n2 % k == 0)
gcd = k; // Update gcd
k++;
}
System.out.println("The greatest common divisor for " + n1 +
" and " + n2 + " is " + gcd);
}
}
Enter first integer: 125
Enter second integer: 2525
The greatest common divisor for 125 and 2525 is 25
Translating a logical solution to Java code is not unique. For example, you could use a for
loop to rewrite the code as follows:
for (int k = 2; k <= n1 && k <= n2; k++) {
if (n1 % k == 0 && n2 % k == 0)
gcd = k;
}
multiple solutions
erroneous solutions
A problem often has multiple solutions, and the gcd problem can be solved in many ways.
Programming Exercise 5.14 suggests another solution. A more efficient solution is to use the
classic Euclidean algorithm (see Section 22.6).
You might think that a divisor for a number n1 cannot be greater than n1 / 2 and would
attempt to improve the program using the following loop:
for (int k = 2; k <= n1 / 2 && k <= n2 / 2; k++) {
if (n1 % k == 0 && n2 % k == 0)
gcd = k;
}
5.8 Case Studies 181
This revision is wrong. Can you find the reason? See Checkpoint Question 5.21 for the
answer.
5.8.2
Case Study: Predicting the Future Tuition
Suppose that the tuition for a university is $10,000 this year and tuition increases 7% every
year. In how many years will the tuition be doubled?
Before you can write a program to solve this problem, first consider how to solve it by
hand. The tuition for the second year is the tuition for the first year * 1.07. The tuition for a
future year is the tuition of its preceding year * 1.07. Thus, the tuition for each year can be
computed as follows:
double tuition = 10000;
tuition = tuition * 1.07;
tuition = tuition * 1.07;
tuition = tuition * 1.07;
...
int year = 0;
year++;
year++;
year++;
//
//
//
//
Year
Year
Year
Year
think before you code
0
1
2
3
Keep computing the tuition for a new year until it is at least 20000. By then you will know
how many years it will take for the tuition to be doubled. You can now translate the logic into
the following loop:
double tuition = 10000;
// Year 0
int year = 0;
while (tuition < 20000) {
tuition = tuition * 1.07;
year++;
}
The complete program is shown in Listing 5.10.
LISTING 5.10 FutureTuition.java
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
public class FutureTuition {
public static void main(String[] args) {
double tuition = 10000;
// Year 0
int year = 0;
while (tuition < 20000) {
tuition = tuition * 1.07;
year++;
}
System.out.println("Tuition will be doubled in "
+ year + " years");
System.out.printf("Tuition will be $%.2f in %1d years",
tuition, year);
}
}
Tuition will be doubled in 11 years
Tuition will be $21048.52 in 11 years
The while loop (lines 5–8) is used to repeatedly compute the tuition for a new year. The
loop terminates when the tuition is greater than or equal to 20000.
loop
next year’s tuition
182 Chapter 5
Loops
5.8.3
Case Study: Converting Decimals to Hexadecimals
Hexadecimals are often used in computer systems programming (see Appendix F for an
introduction to number systems). How do you convert a decimal number to a hexadecimal
number? To convert a decimal number d to a hexadecimal number is to find the hexadecimal
digits hn, hn - 1, hn - 2, c , h2, h1, and h0 such that
d = hn * 16n + hn - 1 * 16n - 1 + hn - 2 * 16n - 2 + g
+ h2 * 162 + h1 * 161 + h0 * 160
These hexadecimal digits can be found by successively dividing d by 16 until the quotient is
0. The remainders are h0, h1, h2, c , hn - 2, hn - 1, and hn. The hexadecimal digits include the
decimal digits 0, 1, 2, 3, 4, 5, 6, 7, 8, and 9, plus A, which is the decimal value 10; B, which
is the decimal value 11; C, which is 12; D, which is 13; E, which is 14; and F, which is 15.
For example, the decimal number 123 is 7B in hexadecimal. The conversion is done as
follows. Divide 123 by 16. The remainder is 11 (B in hexadecimal) and the quotient is 7.
Continue divide 7 by 16. The remainder is 7 and the quotient is 0. Therefore 7B is the
hexadecimal number for 123.
16
0
7
7
16 123
0
7
112
11
h1
h0
Quotient
Remainder
Listing 5.11 gives a program that prompts the user to enter a decimal number and converts
it into a hex number as a string.
LISTING 5.11 Dec2Hex.java
input decimal
decimal to hex
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
import java.util.Scanner;
public class Dec2Hex {
/** Main method */
public static void main(String[] args) {
// Create a Scanner
Scanner input = new Scanner(System.in);
// Prompt the user to enter a decimal integer
System.out.print("Enter a decimal number: ");
int decimal = input.nextInt();
// Convert decimal to hex
String hex = "";
while (decimal != 0) {
int hexValue = decimal % 16;
// Convert a decimal value to a hex digit
char hexDigit = (hexValue <= 9 && hexValue >= 0) ?
(char)(hexValue + '0') : (char)(hexValue - 10 + 'A');
hex = hexDigit + hex;
5.8 Case Studies 183
24
25
26
27
28
29
decimal = decimal / 16;
}
System.out.println("The hex number is " + hex);
get a hex char
}
}
get a letter
Enter a decimal number: 1234
The hex number is 4D2
line#
14
decimal
hex
1234
""
17
iteration 1
"2"
13
23
24
"D2"
4
23
24
D
4
17
iteration 3
2
77
17
iteration 2
hexDigit
2
23
24
hexValue
"4D2"
4
0
The program prompts the user to enter a decimal integer (line 11), converts it to a hex number as a string (lines 14–25), and displays the result (line 27). To convert a decimal to a hex
number, the program uses a loop to successively divide the decimal number by 16 and obtain
its remainder (line 17). The remainder is converted into a hex character (lines 20–21). The
character is then appended to the hex string (line 23). The hex string is initially empty (line
14). Divide the decimal number by 16 to remove a hex digit from the number (line 24). The
loop ends when the remaining decimal number becomes 0.
The program converts a hexValue between 0 and 15 into a hex character. If hexValue is
between 0 and 9, it is converted to (char)(hexValue + '0') (line 21). Recall that when
adding a character with an integer, the character’s Unicode is used in the evaluation. For
example, if hexValue is 5, (char)(hexValue + '0') returns 5. Similarly, if hexValue
is between 10 and 15, it is converted to (char)(hexValue - 10 + 'A') (line 21). For
instance, if hexValue is 11, (char)(hexValue - 10 + 'A') returns B.
5.21
5.22
5.23
Will the program work if n1 and n2 are replaced by n1 / 2 and n2 / 2 in line 17
in Listing 5.9?
In Listing 5.11, why is it wrong if you change the code (char)(hexValue + '0')
to hexValue + '0' in line 21?
In Listing 5.11, how many times the loop body is executed for a decimal number 245
and how many times the loop body is executed for a decimal number 3245?
✓
Check
Point
184 Chapter 5
Loops
5.9 Keywords break and continue
Key
Point
The break and continue keywords provide additional controls in a loop.
Pedagogical Note
Two keywords, break and continue, can be used in loop statements to provide additional controls. Using break and continue can simplify programming in some cases.
Overusing or improperly using them, however, can make programs difficult to read and
debug. (Note to instructors: You may skip this section without affecting students’ understanding of the rest of the book.)
break statement
You have used the keyword break in a switch statement. You can also use break in a loop
to immediately terminate the loop. Listing 5.12 presents a program to demonstrate the effect
of using break in a loop.
LISTING 5.12 TestBreak.java
break
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
public class TestBreak {
public static void main(String[] args) {
int sum = 0;
int number = 0;
while (number < 20) {
number++;
sum += number;
if (sum >= 100)
break;
}
System.out.println("The number is " + number);
System.out.println("The sum is " + sum);
}
}
The number is 14
The sum is 105
The program in Listing 5.12 adds integers from 1 to 20 in this order to sum until sum is
greater than or equal to 100. Without the if statement (line 9), the program calculates the
sum of the numbers from 1 to 20. But with the if statement, the loop terminates when sum
becomes greater than or equal to 100. Without the if statement, the output would be:
The number is 20
The sum is 210
continue statement
You can also use the continue keyword in a loop. When it is encountered, it ends the current iteration and program control goes to the end of the loop body. In other words, continue
breaks out of an iteration while the break keyword breaks out of a loop. Listing 5.13 presents
a program to demonstrate the effect of using continue in a loop.
LISTING 5.13 TestContinue.java
1
2
3
public class TestContinue {
public static void main(String[] args) {
int sum = 0;
5.9 Keywords break and continue 185
4
5
6
7
8
9
10
11
12
13
14
15
int number = 0;
while (number < 20) {
number++;
if (number ==10 || number == 11)
continue;
sum += number;
}
System.out.println("The sum is " + sum);
}
}
The sum is 189
The program in Listing 5.13 adds integers from 1 to 20 except 10 and 11 to sum. With
the if statement in the program (line 8), the continue statement is executed when number
becomes 10 or 11. The continue statement ends the current iteration so that the rest of the
statement in the loop body is not executed; therefore, number is not added to sum when it is
10 or 11. Without the if statement in the program, the output would be as follows:
The sum is 210
In this case, all of the numbers are added to sum, even when number is 10 or 11. Therefore,
the result is 210, which is 21 more than it was with the if statement.
Note
The continue statement is always inside a loop. In the while and do-while loops,
the loop-continuation-condition is evaluated immediately after the continue
statement. In the for loop, the action-after-each-iteration is performed,
then the loop-continuation-condition is evaluated, immediately after the
continue statement.
You can always write a program without using break or continue in a loop (see Checkpoint Question 5.26). In general, though, using break and continue is appropriate if it
simplifies coding and makes programs easier to read.
Suppose you need to write a program to find the smallest factor other than 1 for an integer
n (assume n >= 2). You can write a simple and intuitive code using the break statement as
follows:
int factor = 2;
while (factor <= n) {
if (n % factor == 0)
break;
factor++;
}
System.out.println("The smallest factor other than 1 for "
+ n + " is " + factor);
You may rewrite the code without using break as follows:
boolean found = false;
int factor = 2;
while (factor <= n && !found) {
if (n % factor == 0)
continue
186 Chapter 5
Loops
found = true;
else
factor++;
}
System.out.println("The smallest factor other than 1 for "
+ n + " is " + factor);
Obviously, the break statement makes this program simpler and easier to read in this case.
However, you should use break and continue with caution. Too many break and continue statements will produce a loop with many exit points and make the program difficult
to read.
Note
Some programming languages have a goto statement. The goto statement indiscriminately transfers control to any statement in the program and executes it. This makes
your program vulnerable to errors. The break and continue statements in Java are
different from goto statements. They operate only in a loop or a switch statement.
The break statement breaks out of the loop, and the continue statement breaks out
of the current iteration in the loop.
goto
Note
Programming is a creative endeavor. There are many different ways to write code. In
fact, you can find a smallest factor using a rather simple code as follows:
int factor = 2;
while (factor <= n && n % factor != 0)
factor++;
✓
Check
Point
5.24
What is the keyword break for? What is the keyword continue for? Will the following programs terminate? If so, give the output.
int balance = 10;
while (true) {
if (balance < 9)
break;
balance = balance - 9;
}
int balance = 10;
while (true) {
if (balance < 9)
continue;
balance = balance - 9;
}
System.out.println("Balance is "
+ balance);
System.out.println("Balance is "
+ balance);
(a)
5.25
The for loop on the left is converted into the while loop on the right. What is
wrong? Correct it.
int sum = 0;
for (int i = 0; i < 4; i++) {
if (i % 3 == 0) continue;
sum += i;
}
5.26
(b)
Converted
Wrong conversion
int i = 0, sum = 0;
while (i < 4) {
if (i % 3 == 0) continue;
sum += i;
i++;
}
Rewrite the programs TestBreak and TestContinue in Listings 5.12 and 5.13
without using break and continue.
5.10 Case Study: Checking Palindromes 187
5.27
After the break statement in (a) is executed in the following loop, which statement
is executed? Show the output. After the continue statement in (b) is executed in the
following loop, which statement is executed? Show the output.
for (int i = 1; i < 4; i++) {
for (int j = 1; j < 4; j++) {
if (i * j > 2)
break;
for (int i = 1; i < 4; i++) {
for (int j = 1; j < 4; j++) {
if (i * j > 2)
continue;
System.out.println(i * j);
System.out.println(i * j);
}
}
System.out.println(i);
}
System.out.println(i);
}
(a)
(b)
5.10 Case Study: Checking Palindromes
This section presents a program that checks whether a string is a palindrome.
A string is a palindrome if it reads the same forward and backward. The words “mom,” “dad,”
and “noon,” for instance, are all palindromes.
The problem is to write a program that prompts the user to enter a string and reports
whether the string is a palindrome. One solution is to check whether the first character in the
string is the same as the last character. If so, check whether the second character is the same
as the second-to-last character. This process continues until a mismatch is found or all the
characters in the string are checked, except for the middle character if the string has an odd
number of characters.
Listing 5.14 gives the program.
Key
Point
think before you code
LISTING 5.14 Palindrome.java
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
import java.util.Scanner;
public class Palindrome {
/** Main method */
public static void main(String[] args) {
// Create a Scanner
Scanner input = new Scanner(System.in);
// Prompt the user to enter a string
System.out.print("Enter a string: ");
String s = input.nextLine();
input string
// The index of the first character in the string
int low = 0;
low index
// The index of the last character in the string
int high = s.length() - 1;
high index
boolean isPalindrome = true;
while (low < high) {
if (s.charAt(low) != s.charAt(high)) {
isPalindrome = false;
break;
}
188 Chapter 5
Loops
26
27
28
29
30
31
32
33
34
35
update indices
low++;
high--;
}
if (isPalindrome)
System.out.println(s + " is a palindrome");
else
System.out.println(s + " is not a palindrome");
}
}
Enter a string: noon
noon is a palindrome
Enter a string: moon
moon is not a palindrome
The program uses two variables, low and high, to denote the position of the two characters at the beginning and the end in a string s (lines 14, 17). Initially, low is 0 and high is s.
length() – 1. If the two characters at these positions match, increment low by 1 and decrement high by 1 (lines 26–27). This process continues until (low >= high) or a mismatch
is found (line 21).
The program uses a boolean variable isPalindrome to denote whether the string s is palindrome. Initially, it is set to true (line 19). When a mismatch is discovered (line 21), isPalindrome is to false (line 22) and the loop is terminated with a break statement (line 23).
5.11 Case Study: Displaying Prime Numbers
Key
Point
This section presents a program that displays the first fifty prime numbers in five lines,
each containing ten numbers.
An integer greater than 1 is prime if its only positive divisor is 1 or itself. For example, 2, 3,
5, and 7 are prime numbers, but 4, 6, 8, and 9 are not.
The problem is to display the first 50 prime numbers in five lines, each of which contains
ten numbers. The problem can be broken into the following tasks:
■
Determine whether a given number is prime.
■
For number = 2, 3, 4, 5, 6, . . ., test whether it is prime.
■
Count the prime numbers.
■
Display each prime number, and display ten numbers per line.
Obviously, you need to write a loop and repeatedly test whether a new number is prime.
If the number is prime, increase the count by 1. The count is 0 initially. When it reaches 50,
the loop terminates.
Here is the algorithm for the problem:
Set the number of prime numbers to be printed as
a constant NUMBER_OF_PRIMES;
Use count to track the number of prime numbers and
set an initial count to 0;
Set an initial number to 2;
5.11 Case Study: Displaying Prime Numbers 189
while (count < NUMBER_OF_PRIMES) {
Test whether number is prime;
if number is prime {
Display the prime number and increase the count;
}
Increment number by 1;
}
To test whether a number is prime, check whether it is divisible by 2, 3, 4, and so on up to
number/2. If a divisor is found, the number is not a prime. The algorithm can be described
as follows:
Use a boolean variable isPrime to denote whether
the number is prime; Set isPrime to true initially;
for (int divisor = 2; divisor <= number / 2; divisor++) {
if (number % divisor == 0) {
Set isPrime to false
Exit the loop;
}
}
The complete program is given in Listing 5.15.
LISTING 5.15 PrimeNumber.java
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
public class PrimeNumber {
public static void main(String[] args) {
final int NUMBER_OF_PRIMES = 50; // Number of primes to display
final int NUMBER_OF_PRIMES_PER_LINE = 10; // Display 10 per line
int count = 0; // Count the number of prime numbers
int number = 2; // A number to be tested for primeness
System.out.println("The first 50 prime numbers are \n");
// Repeatedly find prime numbers
while (count < NUMBER_OF_PRIMES) {
// Assume the number is prime
boolean isPrime = true; // Is the current number prime?
// Test whether number is prime
for (int divisor = 2; divisor <= number / 2; divisor++) {
if (number % divisor == 0) { // If true, number is not prime
isPrime = false; // Set isPrime to false
break; // Exit the for loop
}
}
// Display the prime number and increase the count
if (isPrime) {
count++; // Increase the count
if (count % NUMBER_OF_PRIMES_PER_LINE == 0) {
// Display the number and advance to the new line
System.out.println(number);
}
else
System.out.print(number + " ");
count prime numbers
check primeness
exit loop
display if prime
190 Chapter 5
Loops
33
34
35
36
37
38
39
}
// Check if the next number is prime
number++;
}
}
}
The first 50 prime numbers are
2 3 5 7 11 13 17 19 23 29
31 37 41 43 47 53 59 61 67 71
73 79 83 89 97 101 103 107 109 113
127 131 137 139 149 151 157 163 167 173
179 181 191 193 197 199 211 223 227 229
subproblem
This is a complex program for novice programmers. The key to developing a programmatic
solution for this problem, and for many other problems, is to break it into subproblems and
develop solutions for each of them in turn. Do not attempt to develop a complete solution
in the first trial. Instead, begin by writing the code to determine whether a given number is
prime, then expand the program to test whether other numbers are prime in a loop.
To determine whether a number is prime, check whether it is divisible by a number between
2 and number/2 inclusive (lines 16–21). If so, it is not a prime number (line 18); otherwise, it
is a prime number. For a prime number, display it. If the count is divisible by 10 (lines 27–30),
advance to a new line. The program ends when the count reaches 50.
The program uses the break statement in line 19 to exit the for loop as soon as the number is found to be a nonprime. You can rewrite the loop (lines 16–21) without using the break
statement, as follows:
for (int divisor = 2; divisor <= number / 2 && isPrime;
divisor++) {
// If true, the number is not prime
if (number % divisor == 0) {
// Set isPrime to false, if the number is not prime
isPrime = false;
}
}
However, using the break statement makes the program simpler and easier to read in this case.
KEY TERMS
break statement 184
continue statement 184
do-while loop 168
for loop 171
infinite loop 160
input redirection 167
iteration 158
loop 158
loop body 158
nested loop 176
off-by-one error 160
output redirection 167
posttest loop 174
pretest loop 174
sentinel value 165
while loop 158
Programming Exercises 191
CHAPTER SUMMARY
1. There are three types of repetition statements: the while loop, the do-while loop, and
the for loop.
2. The part of the loop that contains the statements to be repeated is called the loop body.
3. A one-time execution of a loop body is referred to as an iteration of the loop.
4. An infinite loop is a loop statement that executes infinitely.
5. In designing loops, you need to consider both the loop control structure and the loop
body.
6. The while loop checks the loop-continuation-condition first. If the condition
is true, the loop body is executed; if it is false, the loop terminates.
7. The do-while loop is similar to the while loop, except that the do-while loop executes the loop body first and then checks the loop-continuation-condition to
decide whether to continue or to terminate.
8. The while loop and the do-while loop often are used when the number of repetitions
is not predetermined.
9. A sentinel value is a special value that signifies the end of the loop.
10. The for loop generally is used to execute a loop body a fixed number of times.
11. The for loop control has three parts. The first part is an initial action that often initializes a control variable. The second part, the loop-continuation-condition,
determines whether the loop body is to be executed. The third part is executed after
each iteration and is often used to adjust the control variable. Usually, the loop control
variables are initialized and changed in the control structure.
12. The while loop and for loop are called pretest loops because the continuation condition is checked before the loop body is executed.
13. The do-while loop is called a posttest loop because the condition is checked after the
loop body is executed.
14. Two keywords, break and continue, can be used in a loop.
15. The break keyword immediately ends the innermost loop, which contains the break.
16. The continue keyword only ends the current iteration.
QUIZ
Answer the quiz for this chapter online at www.cs.armstrong.edu/liang/intro10e/quiz.html.
PROGRAMMING EXERCISES
Pedagogical Note
Read each problem several times until you understand it. Think how to solve the problem before starting to write code. Translate your logic into a program.
A problem often can be solved in many different ways. Students are encouraged to
explore various solutions.
read and think before coding
explore solutions
192 Chapter 5
Loops
Sections 5.2–5.7
*5.1 (Count positive and negative numbers and compute the average of numbers) Write
a program that reads an unspecified number of integers, determines how many
positive and negative values have been read, and computes the total and average of
the input values (not counting zeros). Your program ends with the input 0. Display
the average as a floating-point number. Here is a sample run:
Enter an integer, the input ends if it is 0: 1 2 -1 3 0
The number of positives is 3
The number of negatives is 1
The total is 5.0
The average is 1.25
Enter an integer, the input ends if it is 0: 0
No numbers are entered except 0
5.2 (Repeat additions) Listing 5.4, SubtractionQuizLoop.java, generates five random
5.3
subtraction questions. Revise the program to generate ten random addition questions for two integers between 1 and 15. Display the correct count and test time.
(Conversion from kilograms to pounds) Write a program that displays the following table (note that 1 kilogram is 2.2 pounds):
Kilograms
1
3
...
197
199
Pounds
2.2
6.6
433.4
437.8
5.4 (Conversion from miles to kilometers) Write a program that displays the following table (note that 1 mile is 1.609 kilometers):
Miles
1
2
...
9
10
Kilometers
1.609
3.218
14.481
16.090
5.5 (Conversion from kilograms to pounds and pounds to kilograms) Write a program
that displays the following two tables side by side:
Kilograms
1
3
...
197
199
Pounds
2.2
6.6
|
|
|
Pounds
20
25
433.4
437.8
|
|
510
515
Kilograms
9.09
11.36
231.82
234.09
5.6 (Conversion from miles to kilometers) Write a program that displays the following two tables side by side:
Programming Exercises 193
Miles
1
2
...
9
10
Kilometers
1.609
3.218
|
|
|
Kilometers
20
25
Miles
12.430
15.538
14.481
16.090
|
|
60
65
37.290
40.398
**5.7 (Financial application: compute future tuition) Suppose that the tuition for a uni-
5.8
*5.9
5.10
5.11
5.12
versity is $10,000 this year and increases 5% every year. In one year, the tuition
will be $10,500. Write a program that computes the tuition in ten years and the
total cost of four years’ worth of tuition after the tenth year.
(Find the highest score) Write a program that prompts the user to enter the number of students and each student’s name and score, and finally displays the name
of the student with the highest score.
(Find the two highest scores) Write a program that prompts the user to enter the
number of students and each student’s name and score, and finally displays the
student with the highest score and the student with the second-highest score.
(Find numbers divisible by 5 and 6) Write a program that displays all the numbers from 100 to 1,000, ten per line, that are divisible by 5 and 6. Numbers are
separated by exactly one space.
(Find numbers divisible by 5 or 6, but not both) Write a program that displays
all the numbers from 100 to 200, ten per line, that are divisible by 5 or 6, but not
both. Numbers are separated by exactly one space.
(Find the smallest n such that n2 7 12,000) Use a while loop to find the smallest
integer n such that n2 is greater than 12,000.
5.13 (Find the largest n such that n3 6 12,000) Use a while loop to find the largest
integer n such that n3 is less than 12,000.
Sections 5.8–5.10
*5.14 (Compute the greatest common divisor) Another solution for Listing 5.9 to find
the greatest common divisor of two integers n1 and n2 is as follows: First find d
to be the minimum of n1 and n2, then check whether d, d-1, d-2, . . . , 2, or 1 is
a divisor for both n1 and n2 in this order. The first such common divisor is the
greatest common divisor for n1 and n2. Write a program that prompts the user to
enter two positive integers and displays the gcd.
*5.15 (Display the ASCII character table) Write a program that prints the characters in
the ASCII character table from ! to ~. Display ten characters per line. The ASCII
table is shown in Appendix B. Characters are separated by exactly one space.
*5.16 (Find the factors of an integer) Write a program that reads an integer and displays
**5.17
all its smallest factors in increasing order. For example, if the input integer is
120, the output should be as follows: 2, 2, 2, 3, 5.
(Display pyramid) Write a program that prompts the user to enter an integer from
1 to 15 and displays a pyramid, as shown in the following sample run:
Enter the number of lines: 7
7
6
6
5
5
5
4
4
4
4
3
3
3
3
3
2
2
2
2
2
2
1
1
1
1
1
1
1
2
2
2
2
2
2
3
3
3
3
3
4
4
4
4
5
5
5
6
6
7
194 Chapter 5
Loops
*5.18 (Display four patterns using loops) Use nested loops that display the following
patterns in four separate programs:
Pattern A
Pattern B
1
1 2 3 4 5 6
Pattern C
1 2
1 2 3 4 5
1 2 3
1 2 3 4
1 2 3 4
1 2 3
1 2 3 4 5
1 2
1 2 3 4 5 6
1
Pattern D
1
1 2 3 4 5 6
2 1
1 2 3 4 5
3 2 1
1 2 3 4
4 3 2 1
1 2 3
5 4 3 2 1
1 2
6 5 4 3 2 1
1
**5.19 (Display numbers in a pyramid pattern) Write a nested for loop that prints the
following output:
1
1
1
2
1
1
2
4
2
1
1
2
4
8
4
2
1
1
2
4
8
16
8
4
2
1
1
2
4
8
16
32
16
8
4
2
1
1
2
4
8
16
32
64
32
16
8
4
2
1
2
4
8
16
32
64 128
64
32
16
8
4
2
1
*5.20 (Display prime numbers between 2 and 1,000) Modify Listing 5.15 to display all
the prime numbers between 2 and 1,000, inclusive. Display eight prime numbers
per line. Numbers are separated by exactly one space.
Comprehensive
**5.21 (Financial application: compare loans with various interest rates) Write a program that lets the user enter the loan amount and loan period in number of years
and displays the monthly and total payments for each interest rate starting from
5% to 8%, with an increment of 1/8. Here is a sample run:
Loan Amount: 10000
Number of Years: 5
Interest Rate
Monthly Payment
5.000%
5.125%
5.250%
...
7.875%
8.000%
**5.22
VideoNote
Display loan schedule
Total Payment
188.71
189.29
189.86
11322.74
11357.13
11391.59
202.17
202.76
12129.97
12165.84
For the formula to compute monthly payment, see Listing 2.9, ComputeLoan.java.
(Financial application: loan amortization schedule) The monthly payment for a
given loan pays the principal and the interest. The monthly interest is computed
by multiplying the monthly interest rate and the balance (the remaining principal). The principal paid for the month is therefore the monthly payment minus
the monthly interest. Write a program that lets the user enter the loan amount,
Programming Exercises 195
number of years, and interest rate and displays the amortization schedule for the
loan. Here is a sample run:
Loan Amount: 10000
Number of Years: 1
Annual Interest Rate: 7
Monthly Payment: 865.26
Total Payment: 10383.21
Payment#
1
2
...
11
12
Interest
58.33
53.62
Principal
806.93
811.64
10.0
5.01
855.26
860.25
Balance
9193.07
8381.43
860.27
0.01
Note
The balance after the last payment may not be zero. If so, the last payment should be
the normal monthly payment plus the final balance.
Hint: Write a loop to display the table. Since the monthly payment is the
same for each month, it should be computed before the loop. The balance
is initially the loan amount. For each iteration in the loop, compute the
interest and principal, and update the balance. The loop may look like this:
for (i = 1; i <= numberOfYears * 12; i++) {
interest = monthlyInterestRate * balance;
principal = monthlyPayment - interest;
balance = balance - principal;
System.out.println(i + "\t\t" + interest
+ "\t\t" + principal + "\t\t" + balance);
}
*5.23 (Demonstrate cancellation errors) A cancellation error occurs when you are
manipulating a very large number with a very small number. The large number
may cancel out the smaller number. For example, the result of 100000000.0 +
0.000000001 is equal to 100000000.0. To avoid cancellation errors and obtain
more accurate results, carefully select the order of computation. For example, in
computing the following series, you will obtain more accurate results by computing from right to left rather than from left to right:
1 +
*5.24
1
1
1
+ + c +
n
2
3
Write a program that compares the results of the summation of the preceding
series, computing from left to right and from right to left with n = 50000.
(Sum a series) Write a program to sum the following series:
1
3
5
7
9
11
95
97
+ + + +
+
+ g +
+
3
5
7
9
11
13
97
99
VideoNote
Sum a series
196 Chapter 5
Loops
**5.25 (Compute p) You can approximate p by using the following series:
p = 4¢ 1 -
**5.26
(-1)i + 1
1
1
1
1
1
+ - + + g +
≤
3
5
7
9
11
2i - 1
Write a program that displays the p value for i = 10000, 20000, …, and
100000.
(Compute e) You can approximate e using the following series:
e = 1 +
1
1
1
1
1
+
+
+
+ g +
1!
2!
3!
4!
i!
Write a program that displays the e value for i = 10000, 20000, …, and
100000. (Hint: Because i! = i * (i - 1) * c * 2 * 1, then
1
1
is
i! i(i - 1)!
**5.27
**5.28
Initialize e and item to be 1 and keep adding a new item to e. The new item is
the previous item divided by i for i = 2, 3, 4, . . . .)
(Display leap years) Write a program that displays all the leap years, ten per line,
from 101 to 2100, separated by exactly one space. Also display the number of
leap years in this period.
(Display the first days of each month) Write a program that prompts the user to
enter the year and first day of the year, and displays the first day of each month
in the year. For example, if the user entered the year 2013, and 2 for Tuesday,
January 1, 2013, your program should display the following output:
January 1, 2013 is Tuesday
...
December 1, 2013 is Sunday
**5.29 (Display calendars) Write a program that prompts the user to enter the year and
first day of the year and displays the calendar table for the year on the console. For
example, if the user entered the year 2013, and 2 for Tuesday, January 1, 2013,
your program should display the calendar for each month in the year, as follows:
January 2013
Sun
Mon
Tue
Wed
Thu
Fri
Sat
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
Programming Exercises 197
December 2013
Sun
Mon
Tue
Wed
Thu
Fri
Sat
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
*5.30 (Financial application: compound value) Suppose you save $100 each month
into a savings account with the annual interest rate 5%. So, the monthly interest
rate is 0.05 / 12 = 0.00417. After the first month, the value in the account
becomes
100 * (1 + 0.00417) = 100.417
After the second month, the value in the account becomes
(100 + 100.417) * (1 + 0.00417) = 201.252
After the third month, the value in the account becomes
(100 + 201.252) * (1 + 0.00417) = 302.507
*5.31
and so on.
Write a program that prompts the user to enter an amount (e.g., 100), the annual
interest rate (e.g., 5), and the number of months (e.g., 6) and displays the amount
in the savings account after the given month.
(Financial application: compute CD value) Suppose you put $10,000 into a CD
with an annual percentage yield of 5.75%. After one month, the CD is worth
10000 + 10000 * 5.75 / 1200 = 10047.92
After two months, the CD is worth
10047.91 + 10047.91 * 5.75 / 1200 = 10096.06
After three months, the CD is worth
10096.06 + 10096.06 * 5.75 / 1200 = 10144.44
and so on.
Write a program that prompts the user to enter an amount (e.g., 10000), the
annual percentage yield (e.g., 5.75), and the number of months (e.g., 18) and
displays a table as shown in the sample run.
198 Chapter 5
Loops
Enter the initial deposit amount: 10000
Enter annual percentage yield: 5.75
Enter maturity period (number of months): 18
Month
1
2
...
17
18
CD Value
10047.92
10096.06
10846.57
10898.54
**5.32 (Game: lottery) Revise Listing 3.8, Lottery.java, to generate a lottery of a twodigit number. The two digits in the number are distinct. (Hint: Generate the first
digit. Use a loop to continuously generate the second digit until it is different
from the first digit.)
**5.33 (Perfect number) A positive integer is called a perfect number if it is equal to
***5.34
the sum of all of its positive divisors, excluding itself. For example, 6 is the first
perfect number because 6 = 3 + 2 + 1. The next is 28 = 14 + 7 + 4 + 2
+ 1. There are four perfect numbers less than 10,000. Write a program to find all
these four numbers.
(Game: scissor, rock, paper) Programming Exercise 3.17 gives a program that
plays the scissor-rock-paper game. Revise the program to let the user continuously play until either the user or the computer wins more than two times than its
opponent.
*5.35 (Summation) Write a program to compute the following summation.
1
1 + 22
+
1
22 + 23
+
1
23 + 24
+ c +
1
2624 + 2625
**5.36 (Business application: checking ISBN ) Use loops to simplify Programming
Exercise 3.9.
**5.37 (Decimal to binary) Write a program that prompts the user to enter a decimal
integer and displays its corresponding binary value. Don’t use Java’s Integer
.toBinaryString(int) in this program.
**5.38 (Decimal to octal) Write a program that prompts the user to enter a decimal
integer and displays its corresponding octal value. Don’t use Java’s Integer
.toOctalString(int) in this program.
*5.39 (Financial application: find the sales amount) You have just started a sales job
in a department store. Your pay consists of a base salary and a commission. The
base salary is $5,000. The scheme shown below is used to determine the commission rate.
Sales Amount
$0.01–$5,000
$5,000.01–$10,000
$10,000.01 and above
Commission Rate
8 percent
10 percent
12 percent
Note that this is a graduated rate. The rate for the first $5,000 is at 8%, the next
$5000 is at 10%, and the rest is at 12%. If the sales amount is 25,000, the commission is 5,000 * 8% + 5,000 * 10% + 15,000 * 12% = 2,700.
Programming Exercises 199
5.40
Your goal is to earn $30,000 a year. Write a program that finds the minimum sales
you have to generate in order to make $30,000.
(Simulation: heads or tails) Write a program that simulates flipping a coin one
million times and displays the number of heads and tails.
*5.41 (Occurrence of max numbers) Write a program that reads integers, finds the largest of them, and counts its occurrences. Assume that the input ends with number
0. Suppose that you entered 3 5 2 5 5 5 0; the program finds that the largest
is 5 and the occurrence count for 5 is 4.
(Hint: Maintain two variables, max and count. max stores the current max number, and count stores its occurrences. Initially, assign the first number to max
and 1 to count. Compare each subsequent number with max. If the number is
greater than max, assign it to max and reset count to 1. If the number is equal to
max, increment count by 1.)
Enter numbers: 3 5 2 5 5 5 0
The largest number is 5
The occurrence count of the largest number is 4
*5.42 (Financial application: find the sales amount) Rewrite Programming Exercise
5.39 as follows:
■
■
Use a for loop instead of a do-while loop.
Let the user enter COMMISSION_SOUGHT instead of fixing it as a constant.
*5.43 (Math: combinations) Write a program that displays all possible combinations
for picking two numbers from integers 1 to 7. Also display the total number of
all combinations.
1 2
1 3
...
...
The total number of all combinations is 21
*5.44 (Computer architecture: bit-level operations) A short value is stored in 16 bits.
Write a program that prompts the user to enter a short integer and displays the 16
bits for the integer. Here are sample runs:
Enter an integer: 5
The bits are 0000000000000101
Enter an integer: -5
The bits are 1111111111111011
(Hint: You need to use the bitwise right shift operator (>>) and the bitwise AND
operator (&), which are covered in Appendix G, Bitwise Operations.)
**5.45 (Statistics: compute mean and standard deviation) In business applications, you
are often asked to compute the mean and standard deviation of data. The mean is
simply the average of the numbers. The standard deviation is a statistic that tells
200 Chapter 5
Loops
you how tightly all the various data are clustered around the mean in a set of data.
For example, what is the average age of the students in a class? How close are the
ages? If all the students are the same age, the deviation is 0.
Write a program that prompts the user to enter ten numbers, and displays the
mean and standard deviations of these numbers using the following formula:
n
n
mean =
a xi
i=1
n
n
x1 + x2 + g + xn
=
n
deviation =
2
a xi -
c
¢ a xi ≤
2
i=1
i=1
n
n - 1
Here is a sample run:
Enter ten numbers: 1 2 3 4.5 5.6 6 7 8 9 10
The mean is 5.61
The standard deviation is 2.99794
*5.46 (Reverse a string) Write a program that prompts the user to enter a string and
displays the string in reverse order.
Enter a string: ABCD
The reversed string is DCBA
*5.47 (Business: check ISBN-13) ISBN-13 is a new standard for indentifying books. It
uses 13 digits d1d2d3d4d5d6d7d8d9d10d11d12d13. The last digit d13 is a checksum,
which is calculated from the other digits using the following formula:
10 - (d1 + 3d2 + d3 + 3d4 + d5 + 3d6 + d7 + 3d8 + d9 + 3d10 + d11 + 3d12)%10
If the checksum is 10, replace it with 0. Your program should read the input as a
string. Here are sample runs:
Enter the first 12 digits of an ISBN-13 as a string: 978013213080
The ISBN-13 number is 9780132130806
Enter the first 12 digits of an ISBN-13 as a string: 978013213079
The ISBN-13 number is 9780132130790
Enter the first 12 digits of an ISBN-13 as a string: 97801320
97801320 is an invalid input
*5.48 (Process string) Write a program that prompts the user to enter a string and displays the characters at odd positions. Here is a sample run:
Enter a string: Beijing Chicago
BiigCiao
Programming Exercises 201
*5.49 (Count vowels and consonants) Assume letters A, E, I, O, and U as the vowels.
Write a program that prompts the user to enter a string and displays the number
of vowels and consonants in the string.
Enter a string: Programming is fun
The number of vowels is 5
The number of consonants is 11
*5.50 (Count uppercase letters) Write a program that prompts the user to enter a string
and displays the number of the uppercase letters in the string.
Enter a string: Welcome to Java
The number of uppercase letters is 2
*5.51 (Longest common prefix) Write a program that prompts the user to enter two
strings and displays the largest common prefix of the two strings. Here are some
sample runs:
Enter the first string: Welcome to C++
Enter the second string: Welcome to programming
The common prefix is Welcome to
Enter the first string: Atlanta
Enter the second string: Macon
Atlanta and Macon have no common prefix
This page intentionally left blank
CHAPTER
6
METHODS
Objectives
■
To define methods with formal parameters (§6.2).
■
To invoke methods with actual parameters (i.e., arguments) (§6.2).
■
To define methods with a return value (§6.3).
■
To define methods without a return value (§6.4).
■
To pass arguments by value (§6.5).
■
To develop reusable code that is modular, easy to read, easy to debug,
and easy to maintain (§6.6).
■
To write a method that converts hexadecimals to decimals (§6.7).
■
To use method overloading and understand ambiguous overloading
(§6.8).
■
To determine the scope of variables (§6.9).
■
To apply the concept of method abstraction in software development
(§6.10).
■
To design and implement methods using stepwise refinement (§6.10).
204 Chapter 6
Methods
6.1 Introduction
Key
Point
problem
Methods can be used to define reusable code and organize and simplify coding.
Suppose that you need to find the sum of integers from 1 to 10, from 20 to 37, and from 35
to 49, respectively. You may write the code as follows:
int sum = 0;
for (int i = 1; i <= 10; i++)
sum += i;
System.out.println("Sum from 1 to 10 is " + sum);
sum = 0;
for (int i = 20; i <= 37; i++)
sum += i;
System.out.println("Sum from 20 to 37 is " + sum);
sum = 0;
for (int i = 35; i <= 49; i++)
sum += i;
System.out.println("Sum from 35 to 49 is " + sum);
You may have observed that computing these sums from 1 to 10, from 20 to 37, and from
35 to 49 are very similar except that the starting and ending integers are different. Wouldn’t
it be nice if we could write the common code once and reuse it? We can do so by defining a
method and invoking it.
The preceding code can be simplified as follows:
why methods?
1
2
3
4
5
6
7
8
9
10
11
12
13
define sum method
main method
invoke sum
public static int sum(int i1, int i2) {
int result = 0;
for (int i = i1; i <= i2; i++)
result += i;
return result;
}
public static void main(String[] args) {
System.out.println("Sum from 1 to 10 is " + sum(1, 10));
System.out.println("Sum from 20 to 37 is " + sum(20, 37));
System.out.println("Sum from 35 to 49 is " + sum(35, 49));
}
Lines 1–7 define the method named sum with two parameters i1 and i2. The statements in
the main method invoke sum(1, 10) to compute the sum from 1 to 10, sum(20, 37) to
compute the sum from 20 to 37, and sum(35, 49) to compute the sum from 35 to 49.
A method is a collection of statements grouped together to perform an operation. In earlier chapters you have used predefined methods such as System.out.println, System.exit, Math
.pow, and Math.random. These methods are defined in the Java library. In this chapter, you will
learn how to define your own methods and apply method abstraction to solve complex problems.
method
6.2 Defining a Method
Key
Point
A method definition consists of its method name, parameters, return value type, and body.
The syntax for defining a method is as follows:
modifier returnValueType methodName(list of parameters) {
// Method body;
}
6.2 Defining a Method 205
Let’s look at a method defined to find the larger between two integers. This method, named
max, has two int parameters, num1 and num2, the larger of which is returned by the method.
Figure 6.1 illustrates the components of this method.
Define a method
modifier
method
header
return value
type
Invoke a method
method
formal
name parameters
public static int max(int num1, int num2) {
int z = max(x, y);
int result;
method
body
if (num1 > num2)
result = num1;
else
result = num2;
return result;
parameter list
method
signature
actual parameters
(arguments)
return value
}
FIGURE 6.1 A method definition consists of a method header and a method body.
The method header specifies the modifiers, return value type, method name, and parameters
of the method. The static modifier is used for all the methods in this chapter. The reason for
using it will be discussed in Chapter 8, Objects and Classes.
A method may return a value. The returnValueType is the data type of the value the
method returns. Some methods perform desired operations without returning a value. In this
case, the returnValueType is the keyword void. For example, the returnValueType
is void in the main method, as well as in System.exit, and System.out.println. If
a method returns a value, it is called a value-returning method; otherwise it is called a void
method.
The variables defined in the method header are known as formal parameters or simply
parameters. A parameter is like a placeholder: when a method is invoked, you pass a value
to the parameter. This value is referred to as an actual parameter or argument. The parameter list refers to the method’s type, order, and number of the parameters. The method name
and the parameter list together constitute the method signature. Parameters are optional; that
is, a method may contain no parameters. For example, the Math.random() method has no
parameters.
The method body contains a collection of statements that implement the method. The
method body of the max method uses an if statement to determine which number is larger
and return the value of that number. In order for a value-returning method to return a result, a
return statement using the keyword return is required. The method terminates when a return
statement is executed.
Note
Some programming languages refer to methods as procedures and functions. In those
languages, a value-returning method is called a function and a void method is called a
procedure.
Caution
In the method header, you need to declare each parameter separately. For instance,
max(int num1, int num2) is correct, but max(int num1, num2) is wrong.
method header
modifier
value-returning method
void method
formal parameter
parameter
actual parameter
argument
parameter list
method signature
206 Chapter 6
Methods
Note
We say “define a method” and “declare a variable.” We are making a subtle distinction
here. A definition defines what the defined item is, but a declaration usually involves
allocating memory to store data for the declared item.
define vs. declare
6.3 Calling a Method
Key
Point
Calling a method executes the code in the method.
In a method definition, you define what the method is to do. To execute the method, you have
to call or invoke it. There are two ways to call a method, depending on whether the method
returns a value or not.
If a method returns a value, a call to the method is usually treated as a value. For example,
int larger = max(3, 4);
calls max(3, 4) and assigns the result of the method to the variable larger. Another example of a call that is treated as a value is
System.out.println(max(3, 4));
which prints the return value of the method call max(3, 4).
If a method returns void, a call to the method must be a statement. For example, the
method println returns void. The following call is a statement:
System.out.println("Welcome to Java!");
Note
A value-returning method can also be invoked as a statement in Java. In this case, the
caller simply ignores the return value. This is not often done, but it is permissible if the
caller is not interested in the return value.
When a program calls a method, program control is transferred to the called method. A called
method returns control to the caller when its return statement is executed or when its methodending closing brace is reached.
Listing 6.1 shows a complete program that is used to test the max method.
VideoNote
Define/invoke max method
main method
invoke max
define method
LISTING 6.1 TestMax.java
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
public class TestMax {
/** Main method */
public static void main(String[] args) {
int i = 5;
int j = 2;
int k = max(i, j);
System.out.println("The maximum of " + i +
" and " + j + " is " + k);
}
/** Return the max of two numbers */
public static int max(int num1, int num2) {
int result;
if (num1 > num2)
result = num1;
else
result = num2;
return result;
}
}
6.3 Calling a Method 207
The maximum of 5 and 2 is 5
line#
4
5
i
j
k
num2
5
2
result
5
2
12
Invoking max
num1
13
undefined
16
5
6
5
This program contains the main method and the max method. The main method is just like
any other method except that it is invoked by the JVM to start the program.
The main method’s header is always the same. Like the one in this example, it includes the
modifiers public and static, return value type void, method name main, and a parameter
of the String[] type. String[] indicates that the parameter is an array of String, a subject addressed in Chapter 7.
The statements in main may invoke other methods that are defined in the class that contains
the main method or in other classes. In this example, the main method invokes max(i, j),
which is defined in the same class with the main method.
When the max method is invoked (line 6), variable i’s value 5 is passed to num1, and variable j’s value 2 is passed to num2 in the max method. The flow of control transfers to the max
method, and the max method is executed. When the return statement in the max method is
executed, the max method returns the control to its caller (in this case the caller is the main
method). This process is illustrated in Figure 6.2.
main method
max method
pass the value i
pass the value j
public static void main(String[] args) {
int i = 5;
int j = 2;
int k = max(i, j);
public static int max(int num1, int num2) {
int result;
if (num1 > num2)
result = num1;
else
result = num2;
System.out.println(
"The maximum of " + i +
" and " + j + " is " + k);
}
return result;
}
FIGURE 6.2 When the max method is invoked, the flow of control transfers to it. Once the max method is finished, it
returns control back to the caller.
Caution
A return statement is required for a value-returning method. The method shown
below in (a) is logically correct, but it has a compile error because the Java compiler
thinks that this method might not return a value.
208 Chapter 6
Methods
public static int sign(int n) {
if (n > 0)
return 1;
else if (n == 0)
return 0;
else if (n < 0)
return –1;
Should be
}
public static int sign(int n) {
if (n > 0)
return 1;
else if (n == 0)
return 0;
else
return –1;
}
(a)
(b)
To fix this problem, delete if (n < 0) in (a), so the compiler will see a return
statement to be reached regardless of how the if statement is evaluated.
Note
Methods enable code sharing and reuse. The max method can be invoked from any
class, not just TestMax. If you create a new class, you can invoke the max method
using ClassName.methodName (i.e., TestMax.max).
reusing method
activation record
call stack
Activation record
for the main method
k:
j: 2
i: 5
(a) The main
method is invoked.
Each time a method is invoked, the system creates an activation record (also called an activation frame) that stores parameters and variables for the method and places the activation
record in an area of memory known as a call stack. A call stack is also known as an execution
stack, runtime stack, or machine stack, and it is often shortened to just “the stack.” When a
method calls another method, the caller’s activation record is kept intact, and a new activation
record is created for the new method called. When a method finishes its work and returns to
its caller, its activation record is removed from the call stack.
A call stack stores the activation records in a last-in, first-out fashion: The activation record
for the method that is invoked last is removed first from the stack. For example, suppose
method m1 calls method m2, and m2 calls method m3. The runtime system pushes m1’s activation record into the stack, then m2’s, and then m3’s. After m3 is finished, its activation record is
removed from the stack. After m2 is finished, its activation record is removed from the stack.
After m1 is finished, its activation record is removed from the stack.
Understanding call stacks helps you to comprehend how methods are invoked. The variables defined in the main method in Listing 6.1 are i, j, and k. The variables defined in
the max method are num1, num2, and result. The variables num1 and num2 are defined
in the method signature and are parameters of the max method. Their values are passed
through method invocation. Figure 6.3 illustrates the activation records for method calls in
the stack.
Activation record for
the max method
result:
num2: 2
num1: 5
Activation record for
the max method
result: 5
num2: 2
num1: 5
Activation record for
the main method
k:
j: 2
i: 5
Activation record for
the main method
k:
j: 2
i: 5
(b) The max
method is invoked.
(c) The max method
is being executed.
Activation record
for the main method
k: 5
j: 2
i: 5
(d) The max method is
finished and the return
value is sent to k.
Stack is empty
(e) The main
method is finished.
FIGURE 6.3 When the max method is invoked, the flow of control transfers to the max method. Once the max method is
finished, it returns control back to the caller.
6.4 void Method Example 209
6.4 void Method Example
A void method does not return a value.
The preceding section gives an example of a value-returning method. This section shows how
to define and invoke a void method. Listing 6.2 gives a program that defines a method named
printGrade and invokes it to print the grade for a given score.
Key
Point
VideoNote
LISTING 6.2 TestVoidMethod.java
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
public class TestVoidMethod {
public static void main(String[] args) {
System.out.print("The grade is ");
printGrade(78.5);
System.out.print("The grade is ");
printGrade(59.5);
Use void method
main method
invoke printGrade
printGrade method
}
public static void printGrade(double score) {
if (score >= 90.0) {
System.out.println('A');
}
else if (score >= 80.0) {
System.out.println('B');
}
else if (score >= 70.0) {
System.out.println('C');
}
else if (score >= 60.0) {
System.out.println('D');
}
else {
System.out.println('F');
}
}
}
The grade is C
The grade is F
The printGrade method is a void method because it does not return any value. A call to a
void method must be a statement. Therefore, it is invoked as a statement in line 4 in the main
method. Like any Java statement, it is terminated with a semicolon.
To see the differences between a void and value-returning method, let’s redesign the
printGrade method to return a value. The new method, which we call getGrade, returns
the grade as shown in Listing 6.3.
invoke void method
void vs. value-returned
LISTING 6.3 TestReturnGradeMethod.java
1
2
3
4
5
6
public class TestReturnGradeMethod {
public static void main(String[] args) {
System.out.print("The grade is " + getGrade(78.5));
System.out.print("\nThe grade is " + getGrade(59.5));
}
main method
invoke getGrade
210 Chapter 6
getGrade method
Methods
7
8
9
10
11
12
13
14
15
16
17
18
19
public static char getGrade(double score) {
if (score >= 90.0)
return 'A';
else if (score >= 80.0)
return 'B';
else if (score >= 70.0)
return 'C';
else if (score >= 60.0)
return 'D';
else
return 'F';
}
}
The grade is C
The grade is F
The getGrade method defined in lines 7–18 returns a character grade based on the numeric
score value. The caller invokes this method in lines 3–4.
The getGrade method can be invoked by a caller wherever a character may appear. The
printGrade method does not return any value, so it must be invoked as a statement.
Note
return in void method
A return statement is not needed for a void method, but it can be used for terminating the method and returning to the method’s caller. The syntax is simply
return;
This is not often done, but sometimes it is useful for circumventing the normal flow of
control in a void method. For example, the following code has a return statement to
terminate the method when the score is invalid.
public static void printGrade(double score) {
if (score < 0 || score > 100) {
System.out.println("Invalid score");
return;
}
if (score >= 90.0) {
System.out.println('A');
}
else if (score >= 80.0) {
System.out.println('B');
}
else if (score >= 70.0) {
System.out.println('C');
}
else if (score >= 60.0) {
System.out.println('D');
}
else {
System.out.println('F');
}
}
6.4 void Method Example 211
6.1
6.2
6.3
6.4
6.5
6.6
What are the benefits of using a method?
How do you define a method? How do you invoke a method?
How do you simplify the max method in Listing 6.1 using the conditional operator?
True or false? A call to a method with a void return type is always a statement itself,
but a call to a value-returning method cannot be a statement by itself.
What is the return type of a main method?
What would be wrong with not writing a return statement in a value-returning
method? Can you have a return statement in a void method? Does the return
statement in the following method cause syntax errors?
public static void xMethod(double x, double y) {
System.out.println(x + y);
return x + y;
}
6.7
6.8
Define the terms parameter, argument, and method signature.
Write method headers (not the bodies) for the following methods:
a. Return a sales commission, given the sales amount and the commission rate.
b. Display the calendar for a month, given the month and year.
c. Return a square root of a number.
d. Test whether a number is even, and returning true if it is.
e. Display a message a specified number of times.
f. Return the monthly payment, given the loan amount, number of years, and annual
interest rate.
g. Return the corresponding uppercase letter, given a lowercase letter.
6.9
Identify and correct the errors in the following program:
1
2
3
4
5
6
7
8
9
10
11
12
6.10
public class Test {
public static method1(int n, m) {
n += m;
method2(3.4);
}
public static int method2(int n) {
if (n > 0) return 1;
else if (n == 0) return 0;
else if (n < 0) return –1;
}
}
Reformat the following program according to the programming style and documentation guidelines proposed in Section 1.9, Programming Style and Documentation.
Use the next-line brace style.
public class Test {
public static double method(double i, double j)
{
while (i < j) {
j--;
}
return j;
}
}
✓
Check
Point
212 Chapter 6
Methods
6.5 Passing Arguments by Values
Key
Point
parameter order association
The arguments are passed by value to parameters when invoking a method.
The power of a method is its ability to work with parameters. You can use println to print
any string and max to find the maximum of any two int values. When calling a method, you
need to provide arguments, which must be given in the same order as their respective parameters in the method signature. This is known as parameter order association. For example, the
following method prints a message n times:
public static void nPrintln(String message, int n) {
for (int i = 0; i < n; i++)
System.out.println(message);
}
You can use nPrintln("Hello", 3) to print Hello three times. The nPrintln("Hello",
3) statement passes the actual string parameter Hello to the parameter message, passes 3 to
n, and prints Hello three times. However, the statement nPrintln(3, "Hello") would be
wrong. The data type of 3 does not match the data type for the first parameter, message, nor
does the second argument, Hello, match the second parameter, n.
Caution
The arguments must match the parameters in order, number, and compatible type, as
defined in the method signature. Compatible type means that you can pass an argument
to a parameter without explicit casting, such as passing an int value argument to a
double value parameter.
pass-by-value
When you invoke a method with an argument, the value of the argument is passed to the
parameter. This is referred to as pass-by-value. If the argument is a variable rather than a
literal value, the value of the variable is passed to the parameter. The variable is not affected,
regardless of the changes made to the parameter inside the method. As shown in Listing 6.4,
the value of x (1) is passed to the parameter n to invoke the increment method (line 5). The
parameter n is incremented by 1 in the method (line 10), but x is not changed no matter what
the method does.
LISTING 6.4 Increment.java
invoke increment
increment n
1
2
3
4
5
6
7
8
9
10
11
12
13
public class Increment {
public static void main(String[] args) {
int x = 1;
System.out.println("Before the call, x is " + x);
increment(x);
System.out.println("After the call, x is " + x);
}
public static void increment(int n) {
n++;
System.out.println("n inside the method is " + n);
}
}
Before the call, x is 1
n inside the method is 2
After the call, x is 1
6.5 Passing Arguments by Values 213
Listing 6.5 gives another program that demonstrates the effect of passing by value. The program creates a method for swapping two variables. The swap method is invoked by passing
two arguments. Interestingly, the values of the arguments are not changed after the method
is invoked.
LISTING 6.5 TestPassByValue.java
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
public class TestPassByValue {
/** Main method */
public static void main(String[] args) {
// Declare and initialize variables
int num1 = 1;
int num2 = 2;
System.out.println("Before invoking the swap method, num1 is " +
num1 + " and num2 is " + num2);
// Invoke the swap method to attempt to swap two variables
swap(num1, num2);
System.out.println("After invoking the swap method, num1 is " +
num1 + " and num2 is " + num2);
}
/** Swap two variables */
public static void swap(int n1, int n2) {
System.out.println("\tInside the swap method");
System.out.println("\t\tBefore swapping, n1 is " + n1
+ " and n2 is " + n2);
// Swap n1 with n2
int temp = n1;
n1 = n2;
n2 = temp;
System.out.println("\t\tAfter swapping, n1 is " + n1
+ " and n2 is " + n2);
}
}
Before invoking the swap method, num1 is 1 and num2 is 2
Inside the swap method
Before swapping, n1 is 1 and n2 is 2
After swapping, n1 is 2 and n2 is 1
After invoking the swap method, num1 is 1 and num2 is 2
Before the swap method is invoked (line 12), num1 is 1 and num2 is 2. After the swap method
is invoked, num1 is still 1 and num2 is still 2. Their values have not been swapped. As shown
in Figure 6.4, the values of the arguments num1 and num2 are passed to n1 and n2, but n1 and
n2 have their own memory locations independent of num1 and num2. Therefore, changes in
n1 and n2 do not affect the contents of num1 and num2.
Another twist is to change the parameter name n1 in swap to num1. What effect does this
have? No change occurs, because it makes no difference whether the parameter and the argument have the same name. The parameter is a variable in the method with its own memory
space. The variable is allocated when the method is invoked, and it disappears when the
method is returned to its caller.
false swap
214 Chapter 6
Methods
The values for n1 and n2 are
swapped, but it does not affect
num1 and num2.
The values of num1 and num2 are
passed to n1 and n2.
Activation record for
the main method
Activation record for
the swap method
temp:
n2: 2
n1: 1
Activation record for
the swap method
temp: 1
n2: 1
n1: 2
Activation record for
the main method
Activation record for
the main method
num2: 2
num1: 1
num2: 2
num1: 1
The main method
is invoked.
num2: 2
num1: 1
The swap method
is invoked.
The swap method
is executed.
Activation record for
the main method
Stack is empty
num2: 2
num1: 1
The swap method
is finished.
The main method
is finished.
FIGURE 6.4 The values of the variables are passed to the method’s parameters.
Note
For simplicity, Java programmers often say passing x to y, which actually means passing
the value of argument x to parameter y.
✓
Check
Point
6.11
6.12
How is an argument passed to a method? Can the argument have the same name as
its parameter?
Identify and correct the errors in the following program:
1
2
3
4
5
6
7
8
9
10
11
6.13
public class Test {
public static void main(String[] args) {
nPrintln(5, "Welcome to Java!");
}
public static void nPrintln(String message, int n) {
int n = 1;
for (int i = 0; i < n; i++)
System.out.println(message);
}
}
What is pass-by-value? Show the result of the following programs.
public class Test {
public static void main(String[] args) {
int max = 0;
max(1, 2, max);
System.out.println(max);
}
public static void max(
int value1, int value2, int max) {
if (value1 > value2)
max = value1;
else
max = value2;
}
public class Test {
public static void main(String[] args) {
int i = 1;
while (i <= 6) {
method1(i, 2);
i++;
}
}
public static void method1(
int i, int num) {
for (int j = 1; j <= i; j++) {
System.out.print(num + " ");
num *= 2;
}
}
System.out.println();
}
}
(a)
(b)
6.6 Modularizing Code 215
public class Test {
public static void main(String[] args) {
// Initialize times
int times = 3;
System.out.println("Before the call,"
+ " variable times is " + times);
public class Test {
public static void main(String[] args) {
int i = 0;
while (i <= 4) {
method1(i);
i++;
}
// Invoke nPrintln and display times
nPrintln("Welcome to Java!", times);
System.out.println("After the call,"
+ " variable times is " + times);
System.out.println("i is " + i);
}
}
// Print the message n times
public static void nPrintln(
String message, int n) {
while (n > 0) {
System.out.println("n = " + n);
System.out.println(message);
n--;
}
}
public static void method1(int i) {
do {
if (i % 3 != 0)
System.out.print(i + " ");
i--;
}
while (i >= 1);
System.out.println();
}
}
}
(c)
6.14
(d)
For (a) in the preceding question, show the contents of the activation records in the
call stack just before the method max is invoked, just as max is entered, just before
max is returned, and right after max is returned.
6.6 Modularizing Code
Modularizing makes the code easy to maintain and debug and enables the code to be
reused.
Methods can be used to reduce redundant code and enable code reuse. Methods can also be
used to modularize code and improve the quality of the program.
Listing 5.9 gives a program that prompts the user to enter two integers and displays
their greatest common divisor. You can rewrite the program using a method, as shown in
Listing 6.6.
LISTING 6.6 GreatestCommonDivisorMethod.java
1
2
3
4
5
6
7
8
9
10
11
12
13
14
import java.util.Scanner;
public class GreatestCommonDivisorMethod {
/** Main method */
public static void main(String[] args) {
// Create a Scanner
Scanner input = new Scanner(System.in);
// Prompt the user to enter two integers
System.out.print("Enter first integer: ");
int n1 = input.nextInt();
System.out.print("Enter second integer: ");
int n2 = input.nextInt();
Key
Point
VideoNote
Modularize code
216 Chapter 6
Methods
invoke gcd
compute gcd
return gcd
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
System.out.println("The greatest common divisor for " + n1 +
" and " + n2 + " is " + gcd(n1, n2));
}
/** Return the gcd of two
public static int gcd(int
int gcd = 1; // Initial
int k = 2; // Possible
integers */
n1, int n2) {
gcd is 1
gcd
while (k <= n1 && k <= n2) {
if (n1 % k == 0 && n2 % k == 0)
gcd = k; // Update gcd
k++;
}
return gcd; // Return gcd
}
}
Enter first integer: 45
Enter second integer: 75
The greatest common divisor for 45 and 75 is 15
By encapsulating the code for obtaining the gcd in a method, this program has several
advantages:
1. It isolates the problem for computing the gcd from the rest of the code in the main
method. Thus, the logic becomes clear and the program is easier to read.
2. The errors on computing the gcd are confined in the gcd method, which narrows the
scope of debugging.
3. The gcd method now can be reused by other programs.
Listing 6.7 applies the concept of code modularization to improve Listing 5.15,
PrimeNumber.java.
LISTING 6.7 PrimeNumberMethod.java
invoke printPrimeNumbers
printPrimeNumbers
method
invoke isPrime
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
public class PrimeNumberMethod {
public static void main(String[] args) {
System.out.println("The first 50 prime numbers are \n");
printPrimeNumbers(50);
}
public static void printPrimeNumbers(int numberOfPrimes) {
final int NUMBER_OF_PRIMES_PER_LINE = 10; // Display 10 per line
int count = 0; // Count the number of prime numbers
int number = 2; // A number to be tested for primeness
// Repeatedly find prime numbers
while (count < numberOfPrimes) {
// Print the prime number and increase the count
if (isPrime(number)) {
count++; // Increase the count
6.7 Case Study: Converting Hexadecimals to Decimals 217
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
if (count % NUMBER_OF_PRIMES_PER_LINE == 0) {
// Print the number and advance to the new line
System.out.printf("%-5s\n", number);
}
else
System.out.printf("%-5s", number);
}
// Check whether the next number is prime
number++;
}
}
/** Check whether number is prime */
public static boolean isPrime(int number) {
for (int divisor = 2; divisor <= number / 2; divisor++) {
if (number % divisor == 0) { // If true, number is not prime
return false; // Number is not a prime
}
}
isPrime method
return true; // Number is prime
}
}
The first 50 prime numbers are
2
31
73
127
179
3
37
79
131
181
5
41
83
137
191
7
43
89
139
193
11
47
97
149
197
13
53
101
151
199
17
59
103
157
211
19
61
107
163
223
23
67
109
167
227
29
71
113
173
229
We divided a large problem into two subproblems: determining whether a number is a prime
and printing the prime numbers. As a result, the new program is easier to read and easier to
debug. Moreover, the methods printPrimeNumbers and isPrime can be reused by other
programs.
6.7 Case Study: Converting Hexadecimals to Decimals
This section presents a program that converts a hexadecimal number into a decimal
number.
Listing 5.11, Dec2Hex.java, gives a program that converts a decimal to a hexadecimal. How
would you convert a hex number into a decimal?
Given a hexadecimal number hnhn - 1hn - 2 c h2h1h0, the equivalent decimal value is
hn * 16n + hn - 1 * 16n - 1 + hn - 2 * 16n - 2 + c
+ h2 * 162 + h1 * 161 + h0 * 160
For example, the hex number AB8C is
10 * 163 + 11 * 162 + 8 * 161 + 12 * 160 = 43916
Our program will prompt the user to enter a hex number as a string and convert it into a decimal using the following method:
public static int hexToDecimal(String hex)
Key
Point
218 Chapter 6
Methods
A brute-force approach is to convert each hex character into a decimal number, multiply it by
16i for a hex digit at the i’s position, and then add all the items together to obtain the equivalent decimal value for the hex number.
Note that
hn * 16n + hn - 1 * 16n - 1 + hn - 2 * 16n - 2 + c + h1 * 161 + h0 * 160
= ( c ((hn * 16 + hn - 1) * 16 + hn - 2) * 16 + c + h1) * 16 + h0
This observation, known as the Horner’s algorithm, leads to the following efficient code for
converting a hex string to a decimal number:
int decimalValue = 0;
for (int i = 0; i < hex.length(); i++) {
char hexChar = hex.charAt(i);
decimalValue = decimalValue * 16 + hexCharToDecimal(hexChar);
}
Here is a trace of the algorithm for hex number AB8C:
i
hexChar
hexCharToDecimal
(hexChar)
before the loop
decimalValue
0
after the 1st iteration
0
A
10
10
after the 2nd iteration
1
B
11
10 * 16 + 11
after the 3rd iteration
2
8
8
(10 * 16 + 11) * 16 + 8
after the 4th iteration
3
C
12
((10 * 16 + 11)
* 16 + 8) * 16 + 12
Listing 6.8 gives the complete program.
LISTING 6.8
input string
hex to decimal
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
Hex2Dec.java
import java.util.Scanner;
public class Hex2Dec {
/** Main method */
public static void main(String[] args) {
// Create a Scanner
Scanner input = new Scanner(System.in);
// Prompt the user to enter a string
System.out.print("Enter a hex number: ");
String hex = input.nextLine();
System.out.println("The decimal value for hex number "
+ hex + " is " + hexToDecimal(hex.toUpperCase()));
}
public static int hexToDecimal(String hex) {
int decimalValue = 0;
for (int i = 0; i < hex.length(); i++) {
char hexChar = hex.charAt(i);
decimalValue = decimalValue * 16 + hexCharToDecimal(hexChar);
6.8 Overloading Methods 219
22
23
24
25
26
27
28
29
30
31
32
33
}
return decimalValue;
}
public static int hexCharToDecimal(char ch) {
if (ch >= 'A' && ch <= 'F')
return 10 + ch - 'A';
else // ch is '0', '1', ..., or '9'
return ch - '0';
}
hex char to decimal
check uppercase
}
Enter a hex number: AB8C
The decimal value for hex number AB8C is 43916
Enter a hex number: af71
The decimal value for hex number af71 is 44913
The program reads a string from the console (line 11), and invokes the hexToDecimal method
to convert a hex string to decimal number (line 14). The characters can be in either lowercase
or uppercase. They are converted to uppercase before invoking the hexToDecimal method.
The hexToDecimal method is defined in lines 17–25 to return an integer. The length of
the string is determined by invoking hex.length() in line 19.
The hexCharToDecimal method is defined in lines 27–32 to return a decimal value for
a hex character. The character can be in either lowercase or uppercase. Recall that to subtract
two characters is to subtract their Unicodes. For example, '5' – '0' is 5.
6.8 Overloading Methods
Overloading methods enables you to define the methods with the same name as long
as their signatures are different.
Key
Point
The max method that was used earlier works only with the int data type. But what if you
need to determine which of two floating-point numbers has the maximum value? The solution is to create another method with the same name but different parameters, as shown in the
following code:
public static double max(double num1, double num2) {
if (num1 > num2)
return num1;
else
return num2;
}
If you call max with int parameters, the max method that expects int parameters will be
invoked; if you call max with double parameters, the max method that expects double
parameters will be invoked. This is referred to as method overloading; that is, two methods
have the same name but different parameter lists within one class. The Java compiler determines which method to use based on the method signature.
method overloading
220 Chapter 6
Methods
Listing 6.9 is a program that creates three methods. The first finds the maximum integer,
the second finds the maximum double, and the third finds the maximum among three double
values. All three methods are named max.
LISTING 6.9 TestMethodOverloading.java
overloaded max
overloaded max
overloaded max
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
public class TestMethodOverloading {
/** Main method */
public static void main(String[] args) {
// Invoke the max method with int parameters
System.out.println("The maximum of 3 and 4 is "
+ max(3, 4));
// Invoke the max method with the double parameters
System.out.println("The maximum of 3.0 and 5.4 is "
+ max(3.0, 5.4));
// Invoke the max method with three double parameters
System.out.println("The maximum of 3.0, 5.4, and 10.14 is "
+ max(3.0, 5.4, 10.14));
}
/** Return the max of two int values */
public static int max(int num1, int num2) {
if (num1 > num2)
return num1;
else
return num2;
}
/** Find the max of two double values */
public static double max(double num1, double num2) {
if (num1 > num2)
return num1;
else
return num2;
}
/** Return the max of three double values */
public static double max(double num1, double num2, double num3) {
return max(max(num1, num2), num3);
}
}
The maximum of 3 and 4 is 4
The maximum of 3.0 and 5.4 is 5.4
The maximum of 3.0, 5.4, and 10.14 is 10.14
When calling max(3, 4) (line 6), the max method for finding the maximum of two integers is
invoked. When calling max(3.0, 5.4) (line 10), the max method for finding the maximum
of two doubles is invoked. When calling max(3.0, 5.4, 10.14) (line 14), the max method
for finding the maximum of three double values is invoked.
Can you invoke the max method with an int value and a double value, such as max(2,
2.5)? If so, which of the max methods is invoked? The answer to the first question is yes.
The answer to the second question is that the max method for finding the maximum of two
double values is invoked. The argument value 2 is automatically converted into a double
value and passed to this method.
6.8 Overloading Methods 221
You may be wondering why the method max(double, double) is not invoked for the
call max(3, 4). Both max(double, double) and max(int, int) are possible matches
for max(3, 4). The Java compiler finds the method that best matches a method invocation.
Since the method max(int, int) is a better matches for max(3, 4) than max(double,
double), max(int, int) is used to invoke max(3, 4).
Tip
Overloading methods can make programs clearer and more readable. Methods that perform the same function with different types of parameters should be given the same
name.
Note
Overloaded methods must have different parameter lists. You cannot overload methods
based on different modifiers or return types.
Note
Sometimes there are two or more possible matches for the invocation of a method, but
the compiler cannot determine the best match. This is referred to as ambiguous invocation. Ambiguous invocation causes a compile error. Consider the following code:
ambiguous invocation
public class AmbiguousOverloading {
public static void main(String[] args) {
System.out.println(max(1, 2));
}
public static double max(int num1, double num2) {
if (num1 > num2)
return num1;
else
return num2;
}
public static double max(double num1, int num2) {
if (num1 > num2)
return num1;
else
return num2;
}
}
Both max(int, double) and max(double, int) are possible candidates to match
max(1, 2). Because neither is better than the other, the invocation is ambiguous,
resulting in a compile error.
6.15
6.16
What is method overloading? Is it permissible to define two methods that have the
same name but different parameter types? Is it permissible to define two methods in a
class that have identical method names and parameter lists but different return value
types or different modifiers?
What is wrong in the following program?
public class Test {
public static void method(int x) {
}
public static int method(int y) {
✓
Check
Point
222 Chapter 6
Methods
return y;
}
}
6.17
Given two method definitions,
public static double m(double x, double y)
public static double m(int x, double y)
tell which of the two methods is invoked for:
a. double z = m(4, 5);
b. double z = m(4, 5.4);
c. double z = m(4.5, 5.4);
6.9 The Scope of Variables
Key
Point
scope of variables
local variable
The scope of a variable is the part of the program where the variable can be
referenced.
Section 2.5 introduced the scope of a variable. This section discusses the scope of variables in detail. A variable defined inside a method is referred to as a local variable. The
scope of a local variable starts from its declaration and continues to the end of the block
that contains the variable. A local variable must be declared and assigned a value before
it can be used.
A parameter is actually a local variable. The scope of a method parameter covers the
entire method. A variable declared in the initial-action part of a for-loop header has its
scope in the entire loop. However, a variable declared inside a for-loop body has its scope
limited in the loop body from its declaration to the end of the block that contains the variable,
as shown in Figure 6.5.
The scope of i
The scope of j
public static void method1() {
.
.
for (int i = 1; i < 10; i++) {
.
.
int j;
.
.
.
}
}
FIGURE 6.5 A variable declared in the initial action part of a for-loop header has its scope
in the entire loop.
You can declare a local variable with the same name in different blocks in a method, but
you cannot declare a local variable twice in the same block or in nested blocks, as shown in
Figure 6.6.
6.10 Case Study: Generating Random Characters 223
It is fine to declare i in two
nonnested blocks.
public static void method1() {
int x = 1;
int y = 1;
It is wrong to declare i in two
nested blocks.
public static void method2() {
int i = 1;
int sum = 0;
for (int i = 1; i < 10; i++) {
x += i;
}
for (int i = 1; i < 10; i++) {
y += i;
}
for (int i = 1; i < 10; i++)
sum += i;
}
}
}
FIGURE 6.6 A variable can be declared multiple times in nonnested blocks, but only once in nested blocks.
Caution
Do not declare a variable inside a block and then attempt to use it outside the block.
Here is an example of a common mistake:
for (int i = 0; i < 10; i++) {
}
System.out.println(i);
The last statement would cause a syntax error, because variable i is not defined outside
of the for loop.
6.18
6.19
What is a local variable?
What is the scope of a local variable?
✓
Check
Point
6.10 Case Study: Generating Random Characters
A character is coded using an integer. Generating a random character is to generate
an integer.
Computer programs process numerical data and characters. You have seen many examples
that involve numerical data. It is also important to understand characters and how to process
them. This section presents an example of generating random characters.
As introduced in Section 4.3, every character has a unique Unicode between 0 and FFFF in
hexadecimal (65535 in decimal). To generate a random character is to generate a random integer
between 0 and 65535 using the following expression (note that since 0 <= Math.random() <
1.0, you have to add 1 to 65535):
(int)(Math.random() * (65535 + 1))
Now let’s consider how to generate a random lowercase letter. The Unicodes for lowercase
letters are consecutive integers starting from the Unicode for a, then that for b, c, . . . , and z.
The Unicode for a is
(int)'a'
Thus, a random integer between (int)'a' and (int)'z' is
(int)((int)'a' + Math.random() * ((int)'z' - (int)'a' + 1))
Key
Point
224 Chapter 6
Methods
As discussed in Section 4.3.3, all numeric operators can be applied to the char operands. The
char operand is cast into a number if the other operand is a number or a character. Therefore,
the preceding expression can be simplified as follows:
'a' + Math.random() * ('z' - 'a' + 1)
and a random lowercase letter is
(char)('a' + Math.random() * ('z' - 'a' + 1))
Hence, a random character between any two characters ch1 and ch2 with ch1 < ch2 can be
generated as follows:
(char)(ch1 + Math.random() * (ch2 – ch1 + 1))
This is a simple but useful discovery. Listing 6.10 defines a class named RandomCharacter
with five overloaded methods to get a certain type of character randomly. You can use these
methods in your future projects.
LISTING 6.10 RandomCharacter.java
getRandomCharacter
getRandomLower
CaseLetter()
getRandomUpper
CaseLetter()
getRandomDigit
Character()
getRandomCharacter()
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
public class RandomCharacter {
/** Generate a random character between ch1 and ch2 */
public static char getRandomCharacter(char ch1, char ch2) {
return (char)(ch1 + Math.random() * (ch2 - ch1 + 1));
}
/** Generate a random lowercase letter */
public static char getRandomLowerCaseLetter() {
return getRandomCharacter('a', 'z');
}
/** Generate a random uppercase letter */
public static char getRandomUpperCaseLetter() {
return getRandomCharacter('A', 'Z');
}
/** Generate a random digit character */
public static char getRandomDigitCharacter() {
return getRandomCharacter('0', '9');
}
/** Generate a random character */
public static char getRandomCharacter() {
return getRandomCharacter('\u0000', '\uFFFF');
}
}
Listing 6.11 gives a test program that displays 175 random lowercase letters.
LISTING 6.11 TestRandomCharacter.java
constants
1
2
3
4
5
6
7
8
public class TestRandomCharacter {
/** Main method */
public static void main(String[] args) {
final int NUMBER_OF_CHARS = 175;
final int CHARS_PER_LINE = 25;
// Print random characters between 'a' and 'z', 25 chars per line
for (int i = 0; i < NUMBER_OF_CHARS; i++) {
6.11 Method Abstraction and Stepwise Refinement 225
9
10
11
12
13
14
15
16
char ch = RandomCharacter.getRandomLowerCaseLetter();
if ((i + 1) % CHARS_PER_LINE == 0)
System.out.println(ch);
else
System.out.print(ch);
lower-case letter
}
}
}
gmjsohezfkgtazqgmswfclrao
pnrunulnwmaztlfjedmpchcif
lalqdgivxkxpbzulrmqmbhikr
lbnrjlsopfxahssqhwuuljvbe
xbhdotzhpehbqmuwsfktwsoli
cbuwkzgxpmtzihgatdslvbwbz
bfesoklwbhnooygiigzdxuqni
Line 9 invokes getRandomLowerCaseLetter() defined in the RandomCharacter class.
Note that getRandomLowerCaseLetter() does not have any parameters, but you still have
to use the parentheses when defining and invoking the method.
parentheses required
6.11 Method Abstraction and Stepwise Refinement
The key to developing software is to apply the concept of abstraction.
You will learn many levels of abstraction from this book. Method abstraction is achieved by
separating the use of a method from its implementation. The client can use a method without
knowing how it is implemented. The details of the implementation are encapsulated in the
method and hidden from the client who invokes the method. This is also known as information
hiding or encapsulation. If you decide to change the implementation, the client program will
not be affected, provided that you do not change the method signature. The implementation of
the method is hidden from the client in a “black box,” as shown in Figure 6.7.
Optional arguments
for input
Key
Point
VideoNote
Stepwise refinement
method abstraction
information hiding
Optional return
value
Method Header
Black box
Method Body
FIGURE 6.7 The method body can be thought of as a black box that contains the detailed
implementation for the method.
You have already used the System.out.print method to display a string and the max method
to find the maximum number. You know how to write the code to invoke these methods in your
program, but as a user of these methods, you are not required to know how they are implemented.
The concept of method abstraction can be applied to the process of developing programs.
When writing a large program, you can use the divide-and-conquer strategy, also known
as stepwise refinement, to decompose it into subproblems. The subproblems can be further
decomposed into smaller, more manageable problems.
Suppose you write a program that displays the calendar for a given month of the year. The
program prompts the user to enter the year and the month, then displays the entire calendar for
the month, as shown in the following sample run.
divide and conquer
stepwise refinement
226 Chapter 6
Methods
Enter full year (e.g., 2012): 2012
Enter month as number between 1 and 12: 3
March 2012
----------------------------Sun Mon Tue Wed Thu Fri Sat
1
2
3
4
5
6
7
8
9 10
11 12 13 14 15 16 17
18 19 20 21 22 23 24
25 26 27 28 29 30
Let us use this example to demonstrate the divide-and-conquer approach.
6.11.1
Top-Down Design
How would you get started on such a program? Would you immediately start coding? Beginning programmers often start by trying to work out the solution to every detail. Although
details are important in the final program, concern for detail in the early stages may block
the problem-solving process. To make problem solving flow as smoothly as possible, this
example begins by using method abstraction to isolate details from design and only later
implements the details.
For this example, the problem is first broken into two subproblems: get input from the
user and print the calendar for the month. At this stage, you should be concerned with
what the subproblems will achieve, not with how to get input and print the calendar for the
month. You can draw a structure chart to help visualize the decomposition of the problem
(see Figure 6.8a).
printCalendar
(main)
readInput
printMonth
printMonth
(a)
printMonthTitle
printMonthBody
(b)
FIGURE 6.8 The structure chart shows that the printCalendar problem is divided into two subproblems, readInput
and printMonth in (a), and that printMonth is divided into two smaller subproblems, printMonthTitle and
printMonthBody in (b).
You can use Scanner to read input for the year and the month. The problem of printing the
calendar for a given month can be broken into two subproblems: print the month title and print
the month body, as shown in Figure 6.8b. The month title consists of three lines: month and
year, a dashed line, and the names of the seven days of the week. You need to get the month
name (e.g., January) from the numeric month (e.g., 1). This is accomplished in getMonthName (see Figure 6.9a).
In order to print the month body, you need to know which day of the week is the first day of
the month (getStartDay) and how many days the month has (getNumberOfDaysInMonth),
6.11 Method Abstraction and Stepwise Refinement 227
printMonthBody
printMonthTitle
getMonthName
(a)
getNumberOfDaysInMonth
getStartDay
(b)
FIGURE 6.9 (a) To printMonthTitle, you need getMonthName. (b) The printMonthBody
problem is refined into several smaller problems.
as shown in Figure 6.9b. For example, December 2013 has 31 days, and December 1, 2013,
is a Sunday.
How would you get the start day for the first date in a month? There are several ways to do
so. For now, we’ll use an alternative approach. Assume you know that the start day for January
1, 1800, was a Wednesday (START_DAY_FOR_JAN_1_1800 = 3). You could compute the
total number of days (totalNumberOfDays) between January 1, 1800, and the first date of the
calendar month. The start day for the calendar month is (totalNumberOfDays + START_
DAY_FOR_JAN_1_1800) % 7, since every week has seven days. Thus, the getStartDay
problem can be further refined as getTotalNumberOfDays, as shown in Figure 6.10a.
getStartDay
getTotalNumberOfDays
getNumberOfDaysInMonth
getTotalNumberOfDays
(a)
isLeapYear
(b)
FIGURE 6.10 (a) To getStartDay, you need getTotalNumberOfDays. (b) The
getTotalNumberOfDays problem is refined into two smaller problems.
To get the total number of days, you need to know whether the year is a leap year and the
number of days in each month. Thus, getTotalNumberOfDays can be further refined into
two subproblems: isLeapYear and getNumberOfDaysInMonth, as shown in Figure 6.10b.
The complete structure chart is shown in Figure 6.11.
6.11.2
Top-Down and/or Bottom-Up Implementation
Now we turn our attention to implementation. In general, a subproblem corresponds to a
method in the implementation, although some are so simple that this is unnecessary. You
would need to decide which modules to implement as methods and which to combine with
other methods. Decisions of this kind should be based on whether the overall program will be
easier to read as a result of your choice. In this example, the subproblem readInput can be
simply implemented in the main method.
You can use either a “top-down” or a “bottom-up” approach. The top-down approach
implements one method in the structure chart at a time from the top to the bottom. Stubs—
a simple but incomplete version of a method—can be used for the methods waiting to be
implemented. The use of stubs enables you to quickly build the framework of the program.
Implement the main method first, and then use a stub for the printMonth method. For example,
top-down approach
stub
228 Chapter 6
Methods
printCalendar
(main)
readInput
printMonth
printMonthTitle
getMonthName
printMonthBody
getStartDay
getTotalNumberOfDays
getNumberOfDaysInMonth
isLeapYear
FIGURE 6.11 The structure chart shows the hierarchical relationship of the subproblems in
the program.
let printMonth display the year and the month in the stub. Thus, your program may begin
like this:
public class PrintCalendar {
/** Main method */
public static void main(String[] args) {
Scanner input = new Scanner(System.in);
// Prompt the user to enter year
System.out.print("Enter full year (e.g., 2012): ");
int year = input.nextInt();
// Prompt the user to enter month
System.out.print("Enter month as a number between 1 and 12: ");
int month = input.nextInt();
// Print calendar for the month of the year
printMonth(year, month);
}
/** A stub for printMonth may look like this */
public static void printMonth(int year, int month){
System.out.print(month + " " + year);
}
/** A stub for printMonthTitle may look like this */
public static void printMonthTitle(int year, int month){
}
/** A stub for getMonthBody may look like this */
public static void printMonthBody(int year, int month){
}
6.11 Method Abstraction and Stepwise Refinement 229
/** A stub for getMonthName may look like this */
public static String getMonthName(int month) {
return "January"; // A dummy value
}
/** A stub for getStartDay may look like this */
public static int getStartDay(int year, int month) {
return 1; // A dummy value
}
/** A stub for getTotalNumberOfDays may look like this */
public static int getTotalNumberOfDays(int year, int month) {
return 10000; // A dummy value
}
/** A stub for getNumberOfDaysInMonth may look like this */
public static int getNumberOfDaysInMonth(int year, int month) {
return 31; // A dummy value
}
/** A stub for isLeapYear may look like this */
public static Boolean isLeapYear(int year) {
return true; // A dummy value
}
}
Compile and test the program, and fix any errors. You can now implement the printMonth
method. For methods invoked from the printMonth method, you can again use stubs.
The bottom-up approach implements one method in the structure chart at a time from the
bottom to the top. For each method implemented, write a test program, known as the driver,
to test it. The top-down and bottom-up approaches are equally good: Both approaches implement methods incrementally, help to isolate programming errors, and make debugging easy.
They can be used together.
6.11.3
Implementation Details
The isLeapYear(int year) method can be implemented using the following code from
Section 3.11:
return year % 400 == 0 || (year % 4 == 0 && year % 100 != 0);
Use the following facts to implement getTotalNumberOfDaysInMonth(int year, int
month):
■
January, March, May, July, August, October, and December have 31 days.
■
April, June, September, and November have 30 days.
■
February has 28 days during a regular year and 29 days during a leap year. A regular
year, therefore, has 365 days, a leap year 366 days.
To implement getTotalNumberOfDays(int year, int month), you need to compute
the total number of days (totalNumberOfDays) between January 1, 1800, and the first day
of the calendar month. You could find the total number of days between the year 1800 and the
calendar year and then figure out the total number of days prior to the calendar month in the
calendar year. The sum of these two totals is totalNumberOfDays.
To print a body, first pad some space before the start day and then print the lines for every
week.
The complete program is given in Listing 6.12.
bottom-up approach
driver
230 Chapter 6
Methods
LISTING 6.12 PrintCalendar.java
printMonth
printMonthTitle
getMonthName
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
import java.util.Scanner;
public class PrintCalendar {
/** Main method */
public static void main(String[] args) {
Scanner input = new Scanner(System.in);
// Prompt the user to enter year
System.out.print("Enter full year (e.g., 2012): ");
int year = input.nextInt();
// Prompt the user to enter month
System.out.print("Enter month as a number between 1 and 12: ");
int month = input.nextInt();
// Print calendar for the month of the year
printMonth(year, month);
}
/** Print the calendar for a month in a year */
public static void printMonth(int year, int month) {
// Print the headings of the calendar
printMonthTitle(year, month);
// Print the body of the calendar
printMonthBody(year, month);
}
/** Print the month title, e.g., March 2012 */
public static void printMonthTitle(int year, int month) {
System.out.println("
" + getMonthName(month)
+ " " + year);
System.out.println("-----------------------------");
System.out.println(" Sun Mon Tue Wed Thu Fri Sat");
}
/** Get the English name for the month */
public static String getMonthName(int month) {
String monthName = "";
switch (month) {
case 1: monthName = "January"; break;
case 2: monthName = "February"; break;
case 3: monthName = "March"; break;
case 4: monthName = "April"; break;
case 5: monthName = "May"; break;
case 6: monthName = "June"; break;
case 7: monthName = "July"; break;
case 8: monthName = "August"; break;
case 9: monthName = "September"; break;
case 10: monthName = "October"; break;
case 11: monthName = "November"; break;
case 12: monthName = "December";
}
return monthName;
}
/** Print month body */
6.11 Method Abstraction and Stepwise Refinement 231
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
public static void printMonthBody(int year, int month) {
// Get start day of the week for the first date in the month
int startDay = getStartDay(year, month)
printMonthBody
// Get number of days in the month
int numberOfDaysInMonth = getNumberOfDaysInMonth(year, month);
// Pad space before the first day of the month
int i = 0;
for (i = 0; i < startDay; i++)
System.out.print("
");
for (i = 1; i <= numberOfDaysInMonth; i++) {
System.out.printf("%4d", i);
if ((i + startDay) % 7 == 0)
System.out.println();
}
System.out.println();
}
/** Get the start day of month/1/year */
public static int getStartDay(int year, int month) {
final int START_DAY_FOR_JAN_1_1800 = 3;
// Get total number of days from 1/1/1800 to month/1/year
int totalNumberOfDays = getTotalNumberOfDays(year, month);
getStartDay
// Return the start day for month/1/year
return (totalNumberOfDays + START_DAY_FOR_JAN_1_1800) % 7;
}
/** Get the total number of days since January 1, 1800 */
public static int getTotalNumberOfDays(int year, int month) {
int total = 0;
getTotalNumberOfDays
// Get the total days from 1800 to 1/1/year
for (int i = 1800; i < year; i++)
if (isLeapYear(i))
total = total + 366;
else
total = total + 365;
// Add days from Jan to the month prior to the calendar month
for (int i = 1; i < month; i++)
total = total + getNumberOfDaysInMonth(year, i);
return total;
}
/** Get the number of days in a month */
public static int getNumberOfDaysInMonth(int year, int month) {
if (month == 1 || month == 3 || month == 5 || month == 7 ||
month == 8 || month == 10 || month == 12)
return 31;
if (month == 4 || month == 6 || month == 9 || month == 11)
return 30;
if (month == 2) return isLeapYear(year) ? 29 : 28;
getNumberOfDaysInMonth
232 Chapter 6
Methods
isLeapYear
119
120
121
122
123
124
125
126
127
return 0; // If month is incorrect
}
/** Determine if it is a leap year */
public static boolean isLeapYear(int year) {
return year % 400 == 0 || (year % 4 == 0 && year % 100 != 0);
}
}
The program does not validate user input. For instance, if the user enters either a month not in
the range between 1 and 12 or a year before 1800, the program displays an erroneous calendar. To avoid this error, add an if statement to check the input before printing the calendar.
This program prints calendars for a month but could easily be modified to print calendars
for a whole year. Although it can print months only after January 1800, it could be modified
to print months before 1800.
6.11.4
Benefits of Stepwise Refinement
Stepwise refinement breaks a large problem into smaller manageable subproblems. Each subproblem can be implemented using a method. This approach makes the program easier to
write, reuse, debug, test, modify, and maintain.
Simpler Program
The print calendar program is long. Rather than writing a long sequence of statements in one
method, stepwise refinement breaks it into smaller methods. This simplifies the program and
makes the whole program easier to read and understand.
Reusing Methods
Stepwise refinement promotes code reuse within a program. The isLeapYear method is
defined once and invoked from the getTotalNumberOfDays and getNumberOfDayInMonth
methods. This reduces redundant code.
Easier Developing, Debugging, and Testing
incremental development and
testing
Since each subproblem is solved in a method, a method can be developed, debugged, and tested
individually. This isolates the errors and makes developing, debugging, and testing easier.
When implementing a large program, use the top-down and/or bottom-up approach. Do
not write the entire program at once. Using these approaches seems to take more development
time (because you repeatedly compile and run the program), but it actually saves time and
makes debugging easier.
Better Facilitating Teamwork
When a large problem is divided into subprograms, subproblems can be assigned to different
programmers. This makes it easier for programmers to work in teams.
KEY TERMS
actual parameter 205
ambiguous invocation 221
argument 205
divide and conquer 225
formal parameter (i.e., parameter)
information hiding 225
method 204
method abstraction 225
205
method overloading 219
method signature 205
modifier 205
parameter 205
pass-by-value 212
scope of a variable 222
stepwise refinement 225
stub 227
Chapter Summary 233
CHAPTER SUMMARY
1. Making programs modular and reusable is one of the central goals in software engineering. Java provides many powerful constructs that help to achieve this goal. Methods are
one such construct.
2. The method header specifies the modifiers, return value type, method name, and parameters of the method. The static modifier is used for all the methods in this chapter.
3. A method may return a value. The returnValueType is the data type of the value the
method returns. If the method does not return a value, the returnValueType is the
keyword void.
4. The parameter list refers to the type, order, and number of a method’s parameters. The
method name and the parameter list together constitute the method signature. Parameters are optional; that is, a method doesn’t need to contain any parameters.
5. A return statement can also be used in a void method for terminating the method and
returning to the method’s caller. This is useful occasionally for circumventing the normal flow of control in a method.
6. The arguments that are passed to a method should have the same number, type, and
order as the parameters in the method signature.
7. When a program calls a method, program control is transferred to the called method. A
called method returns control to the caller when its return statement is executed or when
its method-ending closing brace is reached.
8. A value-returning method can also be invoked as a statement in Java. In this case, the
caller simply ignores the return value.
9. A method can be overloaded. This means that two methods can have the same name, as
long as their method parameter lists differ.
10. A variable declared in a method is called a local variable. The scope of a local variable
starts from its declaration and continues to the end of the block that contains the variable. A local variable must be declared and initialized before it is used.
11. Method abstraction is achieved by separating the use of a method from its implementation. The client can use a method without knowing how it is implemented. The details
of the implementation are encapsulated in the method and hidden from the client who
invokes the method. This is known as information hiding or encapsulation.
12. Method abstraction modularizes programs in a neat, hierarchical manner. Programs
written as collections of concise methods are easier to write, debug, maintain, and
modify than would otherwise be the case. This writing style also promotes method
reusability.
13. When implementing a large program, use the top-down and/or bottom-up coding
approach. Do not write the entire program at once. This approach may seem to take
more time for coding (because you are repeatedly compiling and running the program),
but it actually saves time and makes debugging easier.
234 Chapter 6
Methods
QUIZ
Answer the quiz for this chapter online at www.cs.armstrong.edu/liang/intro10e/quiz.html.
PROGRAMMING EXERCISES
Note
A common error for the exercises in this chapter is that students don’t implement
the methods to meet the requirements even though the output from the main program is correct. For an example of this type of error see www.cs.armstrong.edu/liang/
CommonMethodErrorJava.pdf.
Sections 6.2–6.9
6.1
(Math: pentagonal numbers) A pentagonal number is defined as n(3n–1)/2 for
n = 1, 2, . . ., and so on. Therefore, the first few numbers are 1, 5, 12, 22, . . . .
Write a method with the following header that returns a pentagonal number:
public static int getPentagonalNumber(int n)
*6.2
Write a test program that uses this method to display the first 100 pentagonal
numbers with 10 numbers on each line.
(Sum the digits in an integer) Write a method that computes the sum of the digits
in an integer. Use the following method header:
public static int sumDigits(long n)
**6.3
For example, sumDigits(234) returns 9 (2 + 3 + 4). (Hint: Use the % operator to extract digits, and the / operator to remove the extracted digit. For instance,
to extract 4 from 234, use 234 % 10 ( = 4). To remove 4 from 234, use 234 / 10
(= 23). Use a loop to repeatedly extract and remove the digit until all the digits
are extracted. Write a test program that prompts the user to enter an integer and
displays the sum of all its digits.
(Palindrome integer) Write the methods with the following headers
// Return the reversal of an integer, i.e., reverse(456) returns 654
public static int reverse(int number)
// Return true if number is a palindrome
public static boolean isPalindrome(int number)
*6.4
VideoNote
Reverse an integer
Use the reverse method to implement isPalindrome. A number is a palindrome if its reversal is the same as itself. Write a test program that prompts the
user to enter an integer and reports whether the integer is a palindrome.
(Display an integer reversed) Write a method with the following header to display
an integer in reverse order:
public static void reverse(int number)
For example, reverse(3456) displays 6543. Write a test program that prompts
the user to enter an integer and displays its reversal.
*6.5
(Sort three numbers) Write a method with the following header to display three
numbers in increasing order:
public static void displaySortedNumbers(
double num1, double num2, double num3)
Programming Exercises 235
*6.6
Write a test program that prompts the user to enter three numbers and invokes the
method to display them in increasing order.
(Display patterns) Write a method to display a pattern as follows:
1
2 1
3 2 1
...
n n-1 ... 3 2 1
The method header is
public static void displayPattern(int n)
*6.7
(Financial application: compute the future investment value) Write a method that
computes future investment value at a given interest rate for a specified number
of years. The future investment is determined using the formula in Programming
Exercise 2.21.
Use the following method header:
public static double futureInvestmentValue(
double investmentAmount, double monthlyInterestRate, int years)
For example, futureInvestmentValue(10000, 0.05/12, 5) returns
12833.59.
Write a test program that prompts the user to enter the investment amount (e.g.,
1000) and the interest rate (e.g., 9%) and prints a table that displays future value
for the years from 1 to 30, as shown below:
The amount invested: 1000
Annual interest rate: 9
Years
Future Value
1
1093.80
2
1196.41
...
29
13467.25
30
14730.57
6.8
(Conversions between Celsius and Fahrenheit) Write a class that contains the following two methods:
/** Convert from Celsius to Fahrenheit */
public static double celsiusToFahrenheit(double celsius)
/** Convert from Fahrenheit to Celsius */
public static double fahrenheitToCelsius(double fahrenheit)
The formula for the conversion is:
fahrenheit = (9.0 / 5) * celsius + 32
celsius = (5.0 / 9) * (fahrenheit – 32)
236 Chapter 6
Methods
Write a test program that invokes these methods to display the following tables:
Celsius
Fahrenheit
|
Fahrenheit
Celsius
40.0
104.0
|
120.0
48.89
39.0
102.2
|
110.0
43.33
32.0
89.6
|
40.0
4.44
31.0
87.8
|
30.0
-1.11
...
6.9
(Conversions between feet and meters) Write a class that contains the following
two methods:
/** Convert from feet to meters */
public static double footToMeter(double foot)
/** Convert from meters to feet */
public static double meterToFoot(double meter)
The formula for the conversion is:
meter = 0.305 * foot
foot = 3.279 * meter
Write a test program that invokes these methods to display the following tables:
6.10
6.11
Feet
Meters
|
Meters
Feet
1.0
2.0
...
9.0
10.0
0.305
0.610
|
|
20.0
25.0
65.574
81.967
2.745
3.050
|
|
60.0
65.0
196.721
213.115
(Use the isPrime Method) Listing 6.7, PrimeNumberMethod.java, provides the
isPrime(int number) method for testing whether a number is prime. Use this
method to find the number of prime numbers less than 10000.
(Financial application: compute commissions) Write a method that computes the
commission, using the scheme in Programming Exercise 5.39. The header of the
method is as follows:
public static double computeCommission(double salesAmount)
Write a test program that displays the following table:
Sales Amount
Commission
10000
900.0
15000
1500.0
...
95000
11100.0
100000
11700.0
Programming Exercises 237
6.12
(Display characters) Write a method that prints characters using the following
header:
public static void printChars(char ch1, char ch2, int
numberPerLine)
This method prints the characters between ch1 and ch2 with the specified numbers per line. Write a test program that prints ten characters per line from 1 to Z.
Characters are separated by exactly one space.
*6.13
(Sum series) Write a method to compute the following series:
m(i) =
2
i
1
+ + c +
2
3
i + 1
Write a test program that displays the following table:
i
1
2
...
19
20
*6.14
m(i)
0.5000
1.1667
16.4023
17.3546
(Estimate p) p can be computed using the following series:
m(i) = 4¢ 1 -
( -1)i + 1
1
1
1
1
1
+ - + + g+
≤
3
5
7
9
11
2i - 1
Write a method that returns m(i) for a given i and write a test program that displays the following table:
*6.15
i
m(i)
1
101
201
301
401
501
601
701
801
901
4.0000
3.1515
3.1466
3.1449
3.1441
3.1436
3.1433
3.1430
3.1428
3.1427
(Financial application: print a tax table) Listing 3.5 gives a program to compute
tax. Write a method for computing tax using the following header:
public static double computeTax(int status, double taxableIncome)
VideoNote
Estimate p
238 Chapter 6
Methods
Use this method to write a program that prints a tax table for taxable income from
$50,000 to $60,000 with intervals of $50 for all the following statuses:
Taxable
Income
Single
Married Joint
or Qualifying
Widow(er)
Married
Separate
Head of
a House
50000
50050
...
59950
60000
8688
8700
6665
6673
8688
8700
7353
7365
11175
11188
8158
8165
11175
11188
9840
9853
Hint:
round the tax into integers using Math.round
.round(computeTax(status, taxableIncome)).
*6.16
(i.e.,
Math
(Number of days in a year) Write a method that returns the number of days in a
year using the following header:
public static int numberOfDaysInAYear(int year)
Write a test program that displays the number of days in year from 2000 to 2020.
Sections 6.10–6.11
*6.17
(Display matrix of 0s and 1s) Write a method that displays an n-by-n matrix using
the following header:
public static void printMatrix(int n)
Each element is 0 or 1, which is generated randomly. Write a test program that
prompts the user to enter n and displays an n-by-n matrix. Here is a sample run:
Enter n: 3
0 1 0
0 0 0
1 1 1
**6.18 (Check password ) Some websites impose certain rules for passwords. Write a
method that checks whether a string is a valid password. Suppose the password
rules are as follows:
■
■
■
*6.19
A password must have at least eight characters.
A password consists of only letters and digits.
A password must contain at least two digits.
Write a program that prompts the user to enter a password and displays Valid
Password if the rules are followed or Invalid Password otherwise.
(The MyTriangle class) Create a class named MyTriangle that contains the
following two methods:
/** Return true if the sum of any two sides is
* greater than the third side. */
public static boolean isValid(
double side1, double side2, double side3)
Programming Exercises 239
/** Return the area of the triangle. */
public static double area(
double side1, double side2, double side3)
*6.20
Write a test program that reads three sides for a triangle and computes the area if
the input is valid. Otherwise, it displays that the input is invalid. The formula for
computing the area of a triangle is given in Programming Exercise 2.19.
(Count the letters in a string) Write a method that counts the number of letters in
a string using the following header:
public static int countLetters(String s)
Write a test program that prompts the user to enter a string and displays the number of letters in the string.
*6.21
(Phone keypads) The international standard letter/number mapping for telephones
is shown in Programming Exercise 4.15. Write a method that returns a number,
given an uppercase letter, as follows:
int getNumber(char uppercaseLetter)
Write a test program that prompts the user to enter a phone number as a string.
The input number may contain letters. The program translates a letter (uppercase
or lowercase) to a digit and leaves all other characters intact. Here is a sample run
of the program:
Enter a string: 1-800-Flowers
1-800-3569377
Enter a string: 1800flowers
18003569377
**6.22
(Math: approximate the square root) There are several techniques for implementing the sqrt method in the Math class. One such technique is known as the
Babylonian method. It approximates the square root of a number, n, by repeatedly
performing a calculation using the following formula:
nextGuess = (lastGuess + n / lastGuess) / 2
When nextGuess and lastGuess are almost identical, nextGuess is the
approximated square root. The initial guess can be any positive value (e.g., 1).
This value will be the starting value for lastGuess. If the difference between
nextGuess and lastGuess is less than a very small number, such as 0.0001,
you can claim that nextGuess is the approximated square root of n. If not, nextGuess becomes lastGuess and the approximation process continues. Implement the following method that returns the square root of n.
public static double sqrt(long n)
*6.23
(Occurrences of a specified character) Write a method that finds the number of
occurrences of a specified character in a string using the following header:
public static int count(String str, char a)
240 Chapter 6
Methods
For example, count("Welcome", 'e') returns 2. Write a test program that
prompts the user to enter a string followed by a character and displays the number
of occurrences of the character in the string.
Sections 6.10–6.12
**6.24
(Display current date and time) Listing 2.7, ShowCurrentTime.java, displays the
current time. Improve this example to display the current date and time. The calendar example in Listing 6.12, PrintCalendar.java, should give you some ideas on
how to find the year, month, and day.
**6.25
(Convert milliseconds to hours, minutes, and seconds) Write a method that converts milliseconds to hours, minutes, and seconds using the following header:
public static String convertMillis(long millis)
The method returns a string as hours:minutes:seconds. For example,
convertMillis(5500) returns a string 0:0:5, convertMillis(100000) returns
a string 0:1:40, and convertMillis(555550000) returns a string 154:19:10.
Comprehensive
**6.26
(Palindromic prime) A palindromic prime is a prime number and also palindromic. For example, 131 is a prime and also a palindromic prime, as are 313 and
757. Write a program that displays the first 100 palindromic prime numbers. Display 10 numbers per line, separated by exactly one space, as follows:
2 3 5 7 11 101 131 151 181 191
313 353 373 383 727 757 787 797 919 929
...
**6.27
(Emirp) An emirp (prime spelled backward) is a nonpalindromic prime number
whose reversal is also a prime. For example, 17 is a prime and 71 is a prime, so 17
and 71 are emirps. Write a program that displays the first 100 emirps. Display 10
numbers per line, separated by exactly one space, as follows:
13 17 31 37 71 73 79 97 107 113
149 157 167 179 199 311 337 347 359 389
...
**6.28
**6.29
(Mersenne prime) A prime number is called a Mersenne prime if it can be written
in the form 2p - 1 for some positive integer p. Write a program that finds all
Mersenne primes with p … 31 and displays the output as follows:
p
2^p –1
2
3
5
...
3
7
31
(Twin primes) Twin primes are a pair of prime numbers that differ by 2. For example, 3 and 5 are twin primes, 5 and 7 are twin primes, and 11 and 13 are twin primes.
Write a program to find all twin primes less than 1,000. Display the output as follows:
(3, 5)
(5, 7)
...
Programming Exercises 241
**6.30
(Game: craps) Craps is a popular dice game played in casinos. Write a program
to play a variation of the game, as follows:
Roll two dice. Each die has six faces representing values 1, 2, …, and 6, respectively. Check the sum of the two dice. If the sum is 2, 3, or 12 (called craps), you
lose; if the sum is 7 or 11 (called natural), you win; if the sum is another value
(i.e., 4, 5, 6, 8, 9, or 10), a point is established. Continue to roll the dice until either
a 7 or the same point value is rolled. If 7 is rolled, you lose. Otherwise, you win.
Your program acts as a single player. Here are some sample runs.
You rolled 5 + 6 = 11
You win
You rolled 1 + 2 = 3
You lose
You rolled 4 + 4 = 8
point is 8
You rolled 6 + 2 = 8
You win
You rolled 3 + 2 = 5
point is 5
You rolled 2 + 5 = 7
You lose
**6.31
(Financial: credit card number validation) Credit card numbers follow certain patterns. A credit card number must have between 13 and 16 digits. It must start with:
■
■
■
■
4 for Visa cards
5 for Master cards
37 for American Express cards
6 for Discover cards
In 1954, Hans Luhn of IBM proposed an algorithm for validating credit card
numbers. The algorithm is useful to determine whether a card number is entered
correctly or whether a credit card is scanned correctly by a scanner. Credit card
numbers are generated following this validity check, commonly known as the
Luhn check or the Mod 10 check, which can be described as follows (for illustration, consider the card number 4388576018402626):
1. Double every second digit from right to left. If doubling of a digit results in a
two-digit number, add up the two digits to get a single-digit number.
4388576018402626
2*2=4
2*2=4
4*2=8
1*2=2
6 * 2 = 12 (1 + 2 = 3)
5 * 2 = 10 (1 + 0 = 1)
8 * 2 = 16 (1 + 6 = 7)
4*2=8
242 Chapter 6
Methods
2. Now add all single-digit numbers from Step 1.
4 + 4 + 8 + 2 + 3 + 1 + 7 + 8 = 37
3. Add all digits in the odd places from right to left in the card number.
6 + 6 + 0 + 8 + 0 + 7 + 8 + 3 = 38
4. Sum the results from Step 2 and Step 3.
37 + 38 = 75
5. If the result from Step 4 is divisible by 10, the card number is valid; otherwise,
it is invalid. For example, the number 4388576018402626 is invalid, but the
number 4388576018410707 is valid.
Write a program that prompts the user to enter a credit card number as a long
integer. Display whether the number is valid or invalid. Design your program to
use the following methods:
/** Return true if the card number is valid */
public static boolean isValid(long number)
/** Get the result from Step 2 */
public static int sumOfDoubleEvenPlace(long number)
/** Return this number if it is a single digit, otherwise,
* return the sum of the two digits */
public static int getDigit(int number)
/** Return sum of odd-place digits in number */
public static int sumOfOddPlace(long number)
/** Return true if the digit d is a prefix for number */
public static boolean prefixMatched(long number, int d)
/** Return the number of digits in d */
public static int getSize(long d)
/** Return the first k number of digits from number. If the
* number of digits in number is less than k, return number. */
public static long getPrefix(long number, int k)
Here are sample runs of the program: (You may also implement this program by
reading the input as a string and processing the string to validate the credit card.)
Enter a credit card number as a long integer:
4388576018410707
4388576018410707 is valid
Enter a credit card number as a long integer:
4388576018402626
4388576018402626 is invalid
**6.32
**6.33
(Game: chance of winning at craps) Revise Exercise 6.30 to run it 10,000 times
and display the number of winning games.
(Current date and time) Invoking System.currentTimeMillis() returns the
elapsed time in milliseconds since midnight of January 1, 1970. Write a program
that displays the date and time. Here is a sample run:
Current date and time is May 16, 2012 10:34:23
Programming Exercises 243
**6.34
6.35
(Print calendar) Programming Exercise 3.21 uses Zeller’s congruence to calculate the day of the week. Simplify Listing 6.12, PrintCalendar.java, using Zeller’s
algorithm to get the start day of the month.
(Geometry: area of a pentagon) The area of a pentagon can be computed using the
following formula:
Area =
5 * s2
p
4 * tan¢ ≤
5
Write a method that returns the area of a pentagon using the following header:
public static double area(double side)
Write a main method that prompts the user to enter the side of a pentagon and
displays its area. Here is a sample run:
Enter the side: 5.5
The area of the pentagon is 52.04444136781625
*6.36
(Geometry: area of a regular polygon) A regular polygon is an n-sided polygon
in which all sides are of the same length and all angles have the same degree (i.e.,
the polygon is both equilateral and equiangular). The formula for computing the
area of a regular polygon is
Area =
n * s2
p
4 * tan¢ ≤
n
Write a method that returns the area of a regular polygon using the following header:
public static double area(int n, double side)
Write a main method that prompts the user to enter the number of sides and the
side of a regular polygon and displays its area. Here is a sample run:
Enter the number of sides: 5
Enter the side: 6.5
The area of the polygon is 72.69017017488385
6.37
(Format an integer) Write a method with the following header to format the integer with the specified width.
public static String format(int number, int width)
The method returns a string for the number with one or more prefix 0s. The size
of the string is the width. For example, format(34, 4) returns 0034 and format(34, 5) returns 00034. If the number is longer than the width, the method
244 Chapter 6
Methods
returns the string representation for the number. For example, format(34, 1)
returns 34.
*6.38
6.39
Write a test program that prompts the user to enter a number and its width and
displays a string returned by invoking format(number, width).
(Generate random characters) Use the methods in RandomCharacter in Listing
6.10 to print 100 uppercase letters and then 100 single digits, printing ten per line.
(Geometry: point position) Programming Exercise 3.32 shows how to test whether
a point is on the left side of a directed line, on the right, or on the same line. Write
the methods with the following headers:
/** Return true if point (x2, y2) is on the left side of the
* directed line from (x0, y0) to (x1, y1) */
public static boolean leftOfTheLine(double x0, double y0,
double x1, double y1, double x2, double y2)
/** Return true if point (x2, y2) is on the same
* line from (x0, y0) to (x1, y1) */
public static boolean onTheSameLine(double x0, double y0,
double x1, double y1, double x2, double y2)
/** Return true if point (x2, y2) is on the
* line segment from (x0, y0) to (x1, y1) */
public static boolean onTheLineSegment(double x0, double y0,
double x1, double y1, double x2, double y2)
Write a program that prompts the user to enter the three points for p0, p1, and p2
and displays whether p2 is on the left of the line from p0 to p1, right, the same
line, or on the line segment. Here are some sample runs:
Enter three points for p0, p1, and p2: 1 1 2 2 1.5 1.5
(1.5, 1.5) is on the line segment from (1.0, 1.0) to (2.0, 2.0)
Enter three points for p0, p1, and p2: 1 1 2 2 3 3
(3.0, 3.0) is on the same line from (1.0, 1.0) to (2.0, 2.0)
Enter three points for p0, p1, and p2: 1 1 2 2 1 1.5
(1.0, 1.5) is on the left side of the line
from (1.0, 1.0) to (2.0, 2.0)
Enter three points for p0, p1, and p2: 1 1 2 2 1 -1
(1.0, -1.0) is on the right side of the line
from (1.0, 1.0) to (2.0, 2.0)
CHAPTER
7
SINGLE-DIMENSIONAL
ARRAYS
Objectives
■
To describe why arrays are necessary in programming (§7.1).
■
To declare array reference variables and create arrays (§§7.2.1–7.2.2).
■
To obtain array size using arrayRefVar.length and know default
values in an array (§7.2.3).
■
To access array elements using indexes (§7.2.4).
■
To declare, create, and initialize an array using an array initializer (§7.2.5).
■
To program common array operations (displaying arrays, summing
all elements, finding the minimum and maximum elements, random
shuffling, and shifting elements) (§7.2.6).
■
To simplify programming using the for each loops (§7.2.7).
■
To apply arrays in application development (AnalyzeNumbers,
DeckOfCards) (§§7.3–7.4).
■
To copy contents from one array to another (§7.5).
■
To develop and invoke methods with array arguments and return values
(§§7.6–7.8).
■
To define a method with a variable-length argument list (§7.9).
■
To search elements using the linear (§7.10.1) or binary (§7.10.2)
search algorithm.
■
To sort an array using the selection sort approach (§7.11).
■
To use the methods in the java.util.Arrays class (§7.12).
■
To pass arguments to the main method from the command line (§7.13).
246 Chapter 7
Single-Dimensional Arrays
7.1 Introduction
Key
Point
problem
why array?
A single array variable can reference a large collection of data.
Often you will have to store a large number of values during the execution of a program.
Suppose, for instance, that you need to read 100 numbers, compute their average, and find
out how many numbers are above the average. Your program first reads the numbers and
computes their average, then compares each number with the average to determine whether
it is above the average. In order to accomplish this task, the numbers must all be stored in
variables. You have to declare 100 variables and repeatedly write almost identical code
100 times. Writing a program this way would be impractical. So, how do you solve this
problem?
An efficient, organized approach is needed. Java and most other high-level languages provide a data structure, the array, which stores a fixed-size sequential collection of elements of
the same type. In the present case, you can store all 100 numbers into an array and access them
through a single array variable.
This chapter introduces single-dimensional arrays. The next chapter will introduce twodimensional and multidimensional arrays.
7.2 Array Basics
Key
Point
index
Once an array is created, its size is fixed. An array reference variable is used to
access the elements in an array using an index.
An array is used to store a collection of data, but often we find it more useful to think of an
array as a collection of variables of the same type. Instead of declaring individual variables,
such as number0, number1, . . . , and number99, you declare one array variable such as
numbers and use numbers[0], numbers[1], . . . , and numbers[99] to represent individual variables. This section introduces how to declare array variables, create arrays, and
process arrays using indexes.
7.2.1
element type
Declaring Array Variables
To use an array in a program, you must declare a variable to reference the array and specify
the array’s element type. Here is the syntax for declaring an array variable:
elementType[] arrayRefVar;
The elementType can be any data type, and all elements in the array will have the same
data type. For example, the following code declares a variable myList that references an
array of double elements.
double[] myList;
Note
You can also use elementType arrayRefVar[] to declare an array variable. This
style comes from the C/C++ language and was adopted in Java to accommodate C/C++
programmers. The style elementType[] arrayRefVar is preferred.
preferred syntax
7.2.2
null
Creating Arrays
Unlike declarations for primitive data type variables, the declaration of an array variable does
not allocate any space in memory for the array. It creates only a storage location for the reference to an array. If a variable does not contain a reference to an array, the value of the variable
is null. You cannot assign elements to an array unless it has already been created. After an
7.2 Array Basics 247
array variable is declared, you can create an array by using the new operator and assign its
reference to the variable with the following syntax:
arrayRefVar = new elementType[arraySize];
new operator
This statement does two things: (1) it creates an array using new elementType[arraySize];
(2) it assigns the reference of the newly created array to the variable arrayRefVar.
Declaring an array variable, creating an array, and assigning the reference of the array to
the variable can be combined in one statement as:
elementType[] arrayRefVar = new elementType[arraySize];
or
elementType arrayRefVar[] = new elementType[arraySize];
Here is an example of such a statement:
double[] myList = new double[10];
This statement declares an array variable, myList, creates an array of ten elements of
double type, and assigns its reference to myList. To assign values to the elements, use
the syntax:
arrayRefVar[index] = value;
For example, the following code initializes the array.
myList[0]
myList[1]
myList[2]
myList[3]
myList[4]
myList[5]
myList[6]
myList[7]
myList[8]
myList[9]
=
=
=
=
=
=
=
=
=
=
5.6;
4.5;
3.3;
13.2;
4.0;
34.33;
34.0;
45.45;
99.993;
11123;
This array is illustrated in Figure 7.1.
double[] myList = new double[10];
myList reference
Array reference
variable
Array element at
index 5
myList[0]
5.6
myList[1]
4.5
myList[2]
3.3
myList[3]
13.2
myList[4]
4.0
myList[5]
34.33
myList[6]
34.0
myList[7]
45.45
myList[8]
99.993
myList[9]
11123
Element value
FIGURE 7.1 The array myList has ten elements of double type and int indices from 0 to 9.
248 Chapter 7
Single-Dimensional Arrays
Note
An array variable that appears to hold an array actually contains a reference to that array.
Strictly speaking, an array variable and an array are different, but most of the time the
distinction can be ignored. Thus it is all right to say, for simplicity, that myList is an
array, instead of stating, at greater length, that myList is a variable that contains a
reference to an array of ten double elements.
array vs. array variable
7.2.3
array length
default values
When space for an array is allocated, the array size must be given, specifying the number of elements that can be stored in it. The size of an array cannot be changed after the array is created.
Size can be obtained using arrayRefVar.length. For example, myList.length is 10.
When an array is created, its elements are assigned the default value of 0 for the numeric
primitive data types, \u0000 for char types, and false for boolean types.
7.2.4
0 based
indexed variable
Array Size and Default Values
Accessing Array Elements
The array elements are accessed through the index. Array indices are 0 based; that is, they
range from 0 to arrayRefVar.length-1. In the example in Figure 7.1, myList holds ten
double values, and the indices are from 0 to 9.
Each element in the array is represented using the following syntax, known as an indexed
variable:
arrayRefVar[index];
For example, myList[9] represents the last element in the array myList.
Caution
Some programming languages use parentheses to reference an array element, as in
myList(9), but Java uses brackets, as in myList[9].
An indexed variable can be used in the same way as a regular variable. For example, the
following code adds the values in myList[0] and myList[1] to myList[2].
myList[2] = myList[0] + myList[1];
The following loop assigns 0 to myList[0], 1 to myList[1], . . . , and 9 to myList[9]:
for (int i = 0; i < myList.length; i++) {
myList[i] = i;
}
7.2.5
array initializer
Array Initializers
Java has a shorthand notation, known as the array initializer, which combines the declaration,
creation, and initialization of an array in one statement using the following syntax:
elementType[] arrayRefVar = {value0, value1, ..., valuek};
For example, the statement
double[] myList = {1.9, 2.9, 3.4, 3.5};
declares, creates, and initializes the array myList with four elements, which is equivalent to
the following statements:
double[] myList = new double[4];
myList[0] = 1.9;
myList[1] = 2.9;
7.2 Array Basics 249
myList[2] = 3.4;
myList[3] = 3.5;
Caution
The new operator is not used in the array-initializer syntax. Using an array initializer, you
have to declare, create, and initialize the array all in one statement. Splitting it would
cause a syntax error. Thus, the next statement is wrong:
double[] myList;
myList = {1.9, 2.9, 3.4, 3.5};
7.2.6
Processing Arrays
When processing array elements, you will often use a for loop—for two reasons:
■
All of the elements in an array are of the same type. They are evenly processed in the
same fashion repeatedly using a loop.
■
Since the size of the array is known, it is natural to use a for loop.
Assume the array is created as follows:
double[] myList = new double[10];
The following are some examples of processing arrays.
1. Initializing arrays with input values: The following loop initializes the array myList
with user input values.
java.util.Scanner input = new java.util.Scanner(System.in);
System.out.print("Enter " + myList.length + " values: ");
for (int i = 0; i < myList.length; i++)
myList[i] = input.nextDouble();
2. Initializing arrays with random values: The following loop initializes the array myList
with random values between 0.0 and 100.0, but less than 100.0.
for (int i = 0; i < myList.length; i++) {
myList[i] = Math.random() * 100;
}
3. Displaying arrays: To print an array, you have to print each element in the array using a
loop like the following:
for (int i = 0; i < myList.length; i++) {
System.out.print(myList[i] + " ");
}
Tip
For an array of the char[] type, it can be printed using one print statement. For example, the following code displays Dallas:
char[] city = {'D', 'a', 'l', 'l', 'a', 's'};
System.out.println(city);
4. Summing all elements: Use a variable named total to store the sum. Initially total
is 0. Add each element in the array to total using a loop like this:
double total = 0;
for (int i = 0; i < myList.length; i++) {
total += myList[i];
}
print character array
250 Chapter 7
Single-Dimensional Arrays
5. Finding the largest element: Use a variable named max to store the largest element.
Initially max is myList[0]. To find the largest element in the array myList, compare
each element with max, and update max if the element is greater than max.
double max = myList[0];
for (int i = 1; i < myList.length; i++) {
if (myList[i] > max) max = myList[i];
}
6. Finding the smallest index of the largest element: Often you need to locate the largest
element in an array. If an array has multiple elements with the same largest value, find the
smallest index of such an element. Suppose the array myList is {1, 5, 3, 4, 5, 5}. The
largest element is 5 and the smallest index for 5 is 1. Use a variable named max to store
the largest element and a variable named indexOfMax to denote the index of the largest
element. Initially max is myList[0], and indexOfMax is 0. Compare each element in
myList with max, and update max and indexOfMax if the element is greater than max.
double max = myList[0];
int indexOfMax = 0;
for (int i = 1; i < myList.length; i++) {
if (myList[i] > max) {
max = myList[i];
indexOfMax = i;
}
}
Random shuffling
7. Random shuffling: In many applications, you need to randomly reorder the elements
in an array. This is called shuffling. To accomplish this, for each element myList[i],
randomly generate an index j and swap myList[i] with myList[j], as follows:
VideoNote
Random shuffling
myList
for (int i = myList.length – 1; i > 0; i––) {
i
[0] .
// Generate an index j randomly with 0 <= j <= i
int j = (int)(Math.random()
[1]
* (i + 1));
A random index [j] .
// Swap myList[i] with myList[j]
double temp = myList[i];
myList[i] = myList[j];
myList[j] = temp;
[i]
}
swap
8. Shifting elements: Sometimes you need to shift the elements left or right. Here is an
example of shifting the elements one position to the left and filling the last element with
the first element:
double temp = myList[0]; // Retain the first element
// Shift elements left
for (int i = 1; i < myList.length; i++) {
myList[i - 1] = myList[i];
}
myList
// Move the first element to fill in the last position
myList[myList.length - 1] = temp;
9. Simplifying coding: Arrays can be used to greatly simplify coding for certain tasks. For
example, suppose you wish to obtain the English name of a given month by its number.
If the month names are stored in an array, the month name for a given month can be
7.2 Array Basics 251
accessed simply via the index. The following code prompts the user to enter a month
number and displays its month name:
String[] months = {"January", "February", ..., "December"};
System.out.print("Enter a month number (1 to 12): ");
int monthNumber = input.nextInt();
System.out.println("The month is " + months[monthNumber - 1]);
If you didn’t use the months array, you would have to determine the month name using
a lengthy multi-way if-else statement as follows:
if (monthNumber == 1)
System.out.println("The month is January");
else if (monthNumber == 2)
System.out.println("The month is February");
...
else
System.out.println("The month is December");
7.2.7
Foreach Loops
Java supports a convenient for loop, known as a foreach loop, which enables you to traverse
the array sequentially without using an index variable. For example, the following code displays all the elements in the array myList:
for (double e: myList) {
System.out.println(e);
}
You can read the code as “for each element e in myList, do the following.” Note that the
variable, e, must be declared as the same type as the elements in myList.
In general, the syntax for a foreach loop is
for (elementType element: arrayRefVar) {
// Process the element
}
You still have to use an index variable if you wish to traverse the array in a different order or
change the elements in the array.
Caution
Accessing an array out of bounds is a common programming error that throws a runtime
ArrayIndexOutOfBoundsException. To avoid it, make sure that you do not use
an index beyond arrayRefVar.length – 1.
Programmers often mistakenly reference the first element in an array with index 1, but
it should be 0. This is called the off-by-one error. Another common off-by-one error in
a loop is using <= where < should be used. For example, the following loop is wrong.
ArrayIndexOutOfBoundsException
off-by-one error
for (int i = 0; i <= list.length; i++)
System.out.print(list[i] + " ");
The <= should be replaced by <.
7.1
7.2
How do you declare an array reference variable and how do you create an array?
When is the memory allocated for an array?
✓
Check
Point
252 Chapter 7
Single-Dimensional Arrays
7.3
What is the output of the following code?
int x = 30;
int[] numbers = new int[x];
x = 60;
System.out.println("x is " + x);
System.out.println("The size of numbers is " + numbers.length);
7.4
Indicate true or false for the following statements:
■ Every element in an array has the same type.
■ The array size is fixed after an array reference variable is declared.
■
■
7.5
The array size is fixed after it is created.
The elements in an array must be a primitive data type.
Which of the following statements are valid?
int i = new int(30);
double d[] = new double[30];
char[] r = new char(1..30);
int i[] = (3, 4, 3, 2);
float f[] = {2.3, 4.5, 6.6};
char[] c = new char();
7.6
7.7
7.8
How do you access elements in an array?
What is the array index type? What is the lowest index? What is the representation of
the third element in an array named a?
Write statements to do the following:
a. Create an array to hold 10 double values.
b. Assign the value 5.5 to the last element in the array.
c. Display the sum of the first two elements.
d. Write a loop that computes the sum of all elements in the array.
e. Write a loop that finds the minimum element in the array.
f. Randomly generate an index and display the element of this index in the array.
g. Use an array initializer to create another array with the initial values 3.5, 5.5,
4.52, and 5.6.
7.9
7.10
What happens when your program attempts to access an array element with an invalid index?
Identify and fix the errors in the following code:
1
2
3
4
5
6
7
8
7.11
public class Test {
public static void main(String[] args) {
double[100] r;
for (int i = 0; i < r.length(); i++);
r(i) = Math.random * 100;
}
}
What is the output of the following code?
1
2
3
public class Test {
public static void main(String[] args) {
int list[] = {1, 2, 3, 4, 5, 6};
7.3 Case Study: Analyzing Numbers 253
4
5
6
7
8
9
10
for (int i = 1; i < list.length; i++)
list[i] = list[i - 1];
for (int i = 0; i < list.length; i++)
System.out.print(list[i] + " ");
}
}
7.3 Case Study: Analyzing Numbers
The problem is to write a program that finds the number of items above the average of
all items.
Key
Point
Now you can write a program using arrays to solve the problem proposed at the beginning of
this chapter. The problem is to read 100 numbers, get the average of these numbers, and find the
number of the items greater than the average. To be flexible for handling any number of input, we
will let the user enter the number of input, rather than fixing it to 100. Listing 7.1 gives a solution.
LISTING 7.1 AnalyzeNumbers.java
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
public class AnalyzeNumbers {
public static void main(String[] args) {
numbers[0]
java.util.Scanner input = new java.util.Scanner(System.in); numbers[1]:
System.out.print("Enter the number of items: ");
numbers[2]:
int n = input.nextInt();
double [] numbers = new double[n];
double sum = 0;
numbers[i]:
System.out.print("Enter the numbers: ");
for (int i = 0; i < n; i++) {
numbers[i] = input.nextDouble();
sum += numbers[i];
}
numbers[n - 3]:
numbers[n - 2]:
numbers[n - 1]:
double average = sum / n;
int count = 0; // The number of elements above average
for (int i = 0; i < n; i++)
if (numbers[i] > average)
count++;
System.out.println("Average is " + average);
System.out.println("Number of elements above the average is "
+ count);
}
}
Enter the number of items: 10
Enter the numbers: 3.4 5 6 1 6.5 7.8 3.5 8.5 6.3 9.5
Average is 5.75
Number of elements above the average is 6
The program prompts the user to enter the array size (line 5) and creates an array with the
specified size (line 6). The program reads the input, stores numbers into the array (line 11),
adds each number to sum in line 11, and obtains the average (line 15). It then compares
.
.
.
create array
store number in array
get average
above average?
254 Chapter 7
Single-Dimensional Arrays
each number in the array with the average to count the number of values above the average
(lines 17–20).
7.4 Case Study: Deck of Cards
Key
Point
The problem is to create a program that will randomly select four cards from a deck
of cards.
Say you want to write a program that will pick four cards at random from a deck of 52 cards.
All the cards can be represented using an array named deck, filled with initial values 0 to 51,
as follows:
int[] deck = new int[52];
VideoNote
// Initialize cards
for (int i = 0; i < deck.length; i++)
deck[i] = i;
Deck of cards
Card numbers 0 to 12, 13 to 25, 26 to 38, and 39 to 51 represent 13 Spades, 13 Hearts,
13 Diamonds, and 13 Clubs, respectively, as shown in Figure 7.2. cardNumber / 13 determines the suit of the card and cardNumber % 13 determines the rank of the card, as shown
in Figure 7.3. After shuffling the array deck, pick the first four cards from deck. The program
displays the cards from these four card numbers.
0
.
.
.
12
13
.
.
.
25
26
.
.
.
38
39
.
.
.
51
deck
[0] 0
.
.
.
.
.
.
[12] 12
[13] 13
.
.
.
.
.
.
[25] 25
[26] 26
.
.
.
.
.
.
[38] 38
[39] 39
.
.
.
.
.
.
[51] 51
13 Spades ( )
13 Hearts ( )
13 Diamonds ( )
13 Clubs ( )
FIGURE 7.2
Random shuffle
deck
[0] 6
[1] 48
[2] 11
[3] 24
[4] .
[5] .
.
.
.
.
.
.
[25] .
[26] .
.
.
.
.
.
.
[38] .
[39] .
.
.
.
.
.
.
[51] .
Card number 6 is the
7 (6 % 13 = 6) of
Spades (7 / 13 is 0)
Card number 48 is the
10 (48 % 13 = 9) of
Clubs (48 / 13 is 3)
Card number 11 is the
Queen (11 % 13 = 11) of
Spades (11 / 13 is 0)
Card number 24 is the
Queen (24 % 13 = 11) of
Hearts (24 / 13 is 1)
52 cards are stored in an array named deck.
0
Spades
1
Hearts
2
Diamonds
3
Clubs
cardNumber / 13 =
0
Ace
1
2
.
cardNumber % 13 =
FIGURE 7.3 CardNumber identifies a card’s suit and rank number.
.
10
Jack
11
Queen
12
King
7.4 Case Study: Deck of Cards 255
Listing 7.2 gives the solution to the problem.
LISTING 7.2 DeckOfCards.java
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
public class DeckOfCards {
public static void main(String[] args) {
int[] deck = new int[52];
String[] suits = {"Spades", "Hearts", "Diamonds", "Clubs"};
String[] ranks = {"Ace", "2", "3", "4", "5", "6", "7", "8", "9",
"10", "Jack", "Queen", "King"};
// Initialize the cards
for (int i = 0; i < deck.length; i++)
deck[i] = i;
// Shuffle the cards
for (int i = 0; i < deck.length; i++) {
// Generate an index randomly
int index = (int)(Math.random() * deck.length);
int temp = deck[i];
deck[i] = deck[index];
deck[index] = temp;
}
// Display the first four cards
for (int i = 0; i < 4; i++) {
String suit = suits[deck[i] / 13];
String rank = ranks[deck[i] % 13];
System.out.println("Card number " + deck[i] + ": "
+ rank + " of " + suit);
}
}
}
Card
Card
Card
Card
number
number
number
number
6: 7 of Spades
48: 10 of Clubs
11: Queen of Spades
24: Queen of Hearts
The program creates an array suits for four suits (line 4) and an array ranks for 13 cards in
a suit (lines 5–6). Each element in these arrays is a string.
The program initializes deck with values 0 to 51 in lines 9–10. The deck value 0 represents the card Ace of Spades, 1 represents the card 2 of Spades, 13 represents the card Ace of
Hearts, and 14 represents the card 2 of Hearts.
Lines 13–19 randomly shuffle the deck. After a deck is shuffled, deck[i] contains an
arbitrary value. deck[i] / 13 is 0, 1, 2, or 3, which determines the suit (line 23). deck[i]
% 13 is a value between 0 and 12, which determines the rank (line 24). If the suits array is
not defined, you would have to determine the suit using a lengthy multi-way if-else statement as follows:
if (deck[i] / 13 == 0)
System.out.print("suit
else if (deck[i] / 13 ==
System.out.print("suit
else if (deck[i] / 13 ==
System.out.print("suit
else
System.out.print("suit
is Spades");
1)
is Hearts");
2)
is Diamonds");
is Clubs");
create array deck
array of strings
array of strings
initialize deck
shuffle deck
suit of a card
rank of a card
256 Chapter 7
Single-Dimensional Arrays
With suits = {"Spades", "Hearts", "Diamonds", "Clubs"} created in an array,
suits[deck / 13] gives the suit for the deck. Using arrays greatly simplifies the solution
for this program.
✓
Check
Point
7.12
Will the program pick four random cards if you replace lines 22–27 in Listing 7.2
DeckOfCards.java with the following code?
for (int i = 0; i < 4; i++) {
int cardNumber = (int)(Math.random() * deck.length);
String suit = suits[cardNumber / 13];
String rank = ranks[cardNumber % 13];
System.out.println("Card number " + cardNumber + ": "
+ rank + " of " + suit);
}
7.5 Copying Arrays
Key
Point
To copy the contents of one array into another, you have to copy the array’s individual
elements into the other array.
Often, in a program, you need to duplicate an array or a part of an array. In such cases you
could attempt to use the assignment statement (=), as follows:
list2 = list1;
copy reference
garbage collection
However, this statement does not copy the contents of the array referenced by list1 to
list2, but instead merely copies the reference value from list1 to list2. After this statement, list1 and list2 reference the same array, as shown in Figure 7.4. The array previously
referenced by list2 is no longer referenced; it becomes garbage, which will be automatically
collected by the Java Virtual Machine (this process is called garbage collection).
Before the assignment
list2 = list1;
list1
Contents
of list1
list2
After the assignment
list2 = list1;
list1
Contents
of list1
list2
Contents
of list2
Contents
of list2
FIGURE 7.4 Before the assignment statement, list1 and list2 point to separate memory
locations. After the assignment, the reference of the list1 array is passed to list2.
In Java, you can use assignment statements to copy primitive data type variables, but not
arrays. Assigning one array variable to another array variable actually copies one reference to
another and makes both variables point to the same memory location.
There are three ways to copy arrays:
■
Use a loop to copy individual elements one by one.
■
Use the static arraycopy method in the System class.
■
Use the clone method to copy arrays; this will be introduced in Chapter 13, Abstract
Classes and Interfaces.
7.6 Passing Arrays to Methods 257
You can write a loop to copy every element from the source array to the corresponding element
in the target array. The following code, for instance, copies sourceArray to targetArray
using a for loop.
int[] sourceArray = {2, 3, 1, 5, 10};
int[] targetArray = new int[sourceArray.length];
for (int i = 0; i < sourceArray.length; i++) {
targetArray[i] = sourceArray[i];
}
Another approach is to use the arraycopy method in the java.lang.System class to copy
arrays instead of using a loop. The syntax for arraycopy is:
arraycopy method
arraycopy(sourceArray, srcPos, targetArray, tarPos, length);
The parameters srcPos and tarPos indicate the starting positions in sourceArray and
targetArray, respectively. The number of elements copied from sourceArray to targetArray is indicated by length. For example, you can rewrite the loop using the following
statement:
System.arraycopy(sourceArray, 0, targetArray, 0, sourceArray.length);
The arraycopy method does not allocate memory space for the target array. The target array
must have already been created with its memory space allocated. After the copying takes place,
targetArray and sourceArray have the same content but independent memory locations.
Note
The arraycopy method violates the Java naming convention. By convention, this
method should be named arrayCopy (i.e., with an uppercase C).
7.13
Use the arraycopy method to copy the following array to a target array t:
int[] source = {3, 4, 5};
7.14
✓
Check
Point
Once an array is created, its size cannot be changed. Does the following code resize
the array?
int[] myList;
myList = new int[10];
// Sometime later you want to assign a new array to myList
myList = new int[20];
7.6 Passing Arrays to Methods
When passing an array to a method, the reference of the array is passed to the method.
Just as you can pass primitive type values to methods, you can also pass arrays to methods.
For example, the following method displays the elements in an int array:
public static void printArray(int[] array) {
for (int i = 0; i < array.length; i++) {
System.out.print(array[i] + " ");
}
}
You can invoke it by passing an array. For example, the following statement invokes the
printArray method to display 3, 1, 2, 6, 4, and 2.
printArray(new int[]{3, 1, 2, 6, 4, 2});
Key
Point
258 Chapter 7
Single-Dimensional Arrays
Note
The preceding statement creates an array using the following syntax:
new elementType[]{value0, value1, ..., valuek};
There is no explicit reference variable for the array. Such array is called an anonymous
array.
anonymous array
pass-by-value
Java uses pass-by-value to pass arguments to a method. There are important differences
between passing the values of variables of primitive data types and passing arrays.
■
For an argument of a primitive type, the argument’s value is passed.
■
For an argument of an array type, the value of the argument is a reference to an array;
this reference value is passed to the method. Semantically, it can be best described as
pass-by-sharing, that is, the array in the method is the same as the array being passed.
Thus, if you change the array in the method, you will see the change outside the method.
pass-by-sharing
Take the following code, for example:
public class Test {
public static void main(String[] args) {
int x = 1; // x represents an int value
int[] y = new int[10]; // y represents an array of int values
m(x, y); // Invoke m with arguments x and y
System.out.println("x is " + x);
System.out.println("y[0] is " + y[0]);
}
public static void m(int number, int[] numbers) {
number = 1001; // Assign a new value to number
numbers[0] = 5555; // Assign a new value to numbers[0]
}
}
x is 1
y[0] is 5555
You may wonder why after m is invoked, x remains 1, but y[0] become 5555. This is
because y and numbers, although they are independent variables, reference the same array, as
illustrated in Figure 7.5. When m(x, y) is invoked, the values of x and y are passed to number and numbers. Since y contains the reference value to the array, numbers now contains
the same reference value to the same array.
Stack
Activation record for
method m
reference
int[] numbers:
int number: 1
Activation record for the
main method
int[] y: reference
int x: 1
Heap
An array of
ten int
values is
stored here
Arrays are
stored in a
heap.
FIGURE 7.5 The primitive type value in x is passed to number, and the reference value in y
is passed to numbers.
7.6 Passing Arrays to Methods 259
Note
Arrays are objects in Java (objects are introduced in Chapter 9). The JVM stores the
objects in an area of memory called the heap, which is used for dynamic memory
allocation.
heap
Listing 7.3 gives another program that shows the difference between passing a primitive data
type value and an array reference variable to a method.
The program contains two methods for swapping elements in an array. The first method,
named swap, fails to swap two int arguments. The second method, named swapFirstTwoInArray, successfully swaps the first two elements in the array argument.
LISTING 7.3
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
TestPassArray.java
public class TestPassArray {
/** Main method */
public static void main(String[] args) {
int[] a = {1, 2};
// Swap elements using the swap method
System.out.println("Before invoking swap");
System.out.println("array is {" + a[0] + ", " + a[1] + "}");
swap(a[0], a[1]);
System.out.println("After invoking swap");
System.out.println("array is {" + a[0] + ", " + a[1] + "}");
// Swap elements using the swapFirstTwoInArray method
System.out.println("Before invoking swapFirstTwoInArray");
System.out.println("array is {" + a[0] + ", " + a[1] + "}");
swapFirstTwoInArray(a);
System.out.println("After invoking swapFirstTwoInArray");
System.out.println("array is {" + a[0] + ", " + a[1] + "}");
}
/** Swap two variables */
public static void swap(int n1, int n2) {
int temp = n1;
n1 = n2;
n2 = temp;
}
/** Swap the first two elements in the array */
public static void swapFirstTwoInArray(int[] array) {
int temp = array[0];
array[0] = array[1];
array[1] = temp;
}
}
Before invoking swap
array is {1, 2}
After invoking swap
array is {1, 2}
Before invoking swapFirstTwoInArray
array is {1, 2}
After invoking swapFirstTwoInArray
array is {2, 1}
false swap
swap array elements
260 Chapter 7
Single-Dimensional Arrays
As shown in Figure 7.6, the two elements are not swapped using the swap method. However,
they are swapped using the swapFirstTwoInArray method. Since the parameters in the
swap method are primitive type, the values of a[0] and a[1] are passed to n1 and n2 inside
the method when invoking swap(a[0], a[1]). The memory locations for n1 and n2 are
independent of the ones for a[0] and a[1]. The contents of the array are not affected by
this call.
Stack
Activation record for
the swap method
n2: 2
n1: 1
Activation record for
the main method
int[] a reference
Invoke swap(int n1, int n2).
The primitive type values in
a[0] and a[1] are passed to the
swap method.
Heap
a[0]: 1
a[1]: 2
The arrays are
stored in a
heap.
Stack
Activation record for the
swapFirstTwoInArray
method
int[] array reference
Activation record for the
main method
int[] a reference
Invoke swapFirstTwoInArray(int[]
array). The reference value in a is passed
to the swapFirstTwoInArray method.
FIGURE 7.6 When passing an array to a method, the reference of the array is passed
to the method.
The parameter in the swapFirstTwoInArray method is an array. As shown in Figure 7.6,
the reference of the array is passed to the method. Thus the variables a (outside the method)
and array (inside the method) both refer to the same array in the same memory location.
Therefore, swapping array[0] with array[1] inside the method swapFirstTwoInArray
is the same as swapping a[0] with a[1] outside of the method.
7.7 Returning an Array from a Method
Key
Point
create array
return array
When a method returns an array, the reference of the array is returned.
You can pass arrays when invoking a method. A method may also return an array. For example, the following method returns an array that is the reversal of another array.
l public static int[] reverse(int[] list) {
2
int[] result = new int[list.length];
3
4
for (int i = 0, j = result.length - 1;
5
i < list.length; i++, j--) {
6
result[j] = list[i];
list
7
}
8
result
9
return result;
10 }
Line 2 creates a new array result. Lines 4–7 copy elements from array list to array
result. Line 9 returns the array. For example, the following statement returns a new array
list2 with elements 6, 5, 4, 3, 2, 1.
int[] list1 = {1, 2, 3, 4, 5, 6};
int[] list2 = reverse(list1);
7.8 Case Study: Counting the Occurrences of Each Letter 261
7.15
Suppose the following code is written to reverse the contents in an array, explain
why it is wrong. How do you fix it?
int[] list = {1, 2, 3, 5, 4};
✓
Check
Point
for (int i = 0, j = list.length - 1; i < list.length; i++, j--) {
// Swap list[i] with list[j]
int temp = list[i];
list[i] = list[j];
list[j] = temp;
}
7.8 Case Study: Counting the Occurrences
of Each Letter
This section presents a program to count the occurrences of each letter in an array of
characters.
Key
Point
The program given in Listing 7.4 does the following:
1. Generates 100 lowercase letters randomly and assigns them to an array of characters, as
shown in Figure 7.7a. You can obtain a random letter by using the getRandomLowerCaseLetter() method in the RandomCharacter class in Listing 6.10.
2. Count the occurrences of each letter in the array. To do so, create an array, say counts,
of 26 int values, each of which counts the occurrences of a letter, as shown in
Figure 7.7b. That is, counts[0] counts the number of a’s, counts[1] counts the
number of b’s, and so on.
chars[0]
counts[0]
chars[1]
counts[1]
…
…
…
…
…
…
…
…
chars[98]
counts[24]
chars[99]
counts[25]
(a)
(b)
FIGURE 7.7 The chars array stores 100 characters, and the counts array stores 26 counts,
each of which counts the occurrences of a letter.
LISTING 7.4 CountLettersInArray.java
1
2
3
4
5
6
7
8
9
10
public class CountLettersInArray {
/** Main method */
public static void main(String[] args) {
// Declare and create an array
char[] chars = createArray();
// Display the array
System.out.println("The lowercase letters are:");
displayArray(chars);
create array
pass array
262 Chapter 7
return array
pass array
increase count
Single-Dimensional Arrays
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
// Count the occurrences of each letter
int[] counts = countLetters(chars);
// Display counts
System.out.println();
System.out.println("The occurrences of each letter are:");
displayCounts(counts);
}
/** Create an array of characters */
public static char[] createArray() {
// Declare an array of characters and create it
char[] chars = new char[100];
// Create lowercase letters randomly and assign
// them to the array
for (int i = 0; i < chars.length; i++)
chars[i] = RandomCharacter.getRandomLowerCaseLetter();
// Return the array
return chars;
}
/** Display the array of characters */
public static void displayArray(char[] chars) {
// Display the characters in the array 20 on each line
for (int i = 0; i < chars.length; i++) {
if ((i + 1) % 20 == 0)
System.out.println(chars[i]);
else
System.out.print(chars[i] + " ");
}
}
/** Count the occurrences of each letter */
public static int[] countLetters(char[] chars) {
// Declare and create an array of 26 int
int[] counts = new int[26];
// For each lowercase letter in the array, count it
for (int i = 0; i < chars.length; i++)
counts[chars[i] - 'a']++;
return counts;
}
/** Display counts */
public static void displayCounts(int[] counts) {
for (int i = 0; i < counts.length; i++) {
if ((i + 1) % 10 == 0)
System.out.println(counts[i] + " " + (char)(i + 'a'));
else
System.out.print(counts[i] + " " + (char)(i + 'a') + " ");
}
}
}
7.8 Case Study: Counting the Occurrences of Each Letter 263
The
e y
s c
a z
h w
q e
lowercase
l s r i b
c k r d w
g d e g f
i w n t g
a m f w p
letters
k j v j
a m p w
i n d x
x w c d
g u q t
The
5 a
2 k
3 u
occurrences
3 b 4 c 4 d
3 l 4 m 6 n
5 v 8 w 3 x
are:
h a b
v u n
m z o
o t x
r e n
z
q
u
h
n
n
a
l
y
w
w
m
o
v
f
b
p
z
z
c
t
l
j
y
r
v
o
v
z
f
of each letter are:
4 e 4 f 4 g 3 h 3 i 3 j
4 o 3 p 3 q 4 r 2 s 4 t
3 y 6 z
The createArray method (lines 21–32) generates an array of 100 random lowercase letters. Line 5 invokes the method and assigns the array to chars. What would be wrong if you
rewrote the code as follows?
char[] chars = new char[100];
chars = createArray();
You would be creating two arrays. The first line would create an array by using new char[100].
The second line would create an array by invoking createArray() and assign the reference
of the array to chars. The array created in the first line would be garbage because it is no longer
referenced, and as mentioned earlier Java automatically collects garbage behind the scenes.
Your program would compile and run correctly, but it would create an array unnecessarily.
Invoking getRandomLowerCaseLetter() (line 28) returns a random lowercase letter.
This method is defined in the RandomCharacter class in Listing 6.10.
The countLetters method (lines 46–55) returns an array of 26 int values, each of
which stores the number of occurrences of a letter. The method processes each letter in the
array and increases its count by one. A brute-force approach to count the occurrences of each
letter might be as follows:
for (int i = 0; i < chars.length; i++)
if (chars[i] == 'a')
counts[0]++;
else if (chars[i] == 'b')
counts[1]++;
...
But a better solution is given in lines 51–52.
for (int i = 0; i < chars.length; i++)
counts[chars[i] - 'a']++;
If the letter (chars[i]) is a, the corresponding count is counts['a' - 'a'] (i.e.,
counts[0]). If the letter is b, the corresponding count is counts['b' - 'a'] (i.e.,
counts[1]), since the Unicode of b is one more than that of a. If the letter is z, the corresponding count is counts['z' - 'a'] (i.e., counts[25]), since the Unicode of z is 25
more than that of a.
Figure 7.8 shows the call stack and heap during and after executing createArray. See
Checkpoint Question 7.18 to show the call stack and heap for other methods in the program.
264 Chapter 7
Single-Dimensional Arrays
Stack
Activation record for the
createArray method
char[] chars: ref
Heap
Stack
Array of 100
characters
Heap
Array of 100
characters
Activation record for the
main method
char[] chars: ref
Activation record for the
main method
char[] chars: ref
(a) Executing
createArray in line 5
(b) After exiting
createArray in line 5
FIGURE 7.8 (a) An array of 100 characters is created when executing createArray.
(b) This array is returned and assigned to the variable chars in the main method.
✓
Check
Point
7.16
True or false? When an array is passed to a method, a new array is created and passed
to the method.
7.17
Show the output of the following two programs:
public class Test {
public static void main(String[] args) {
int number = 0;
int[] numbers = new int[1];
m(number, numbers);
public class Test {
public static void main(String[] args) {
int[] list = {1, 2, 3, 4, 5};
reverse(list);
for (int i = 0; i < list.length; i++)
System.out.print(list[i] + " ");
}
System.out.println("number is " + number
+ " and numbers[0] is " + numbers[0]);
public static void reverse(int[] list) {
int[] newList = new int[list.length];
}
public static void m(int x, int[] y) {
x = 3;
y[0] = 3;
}
for (int i = 0; i < list.length; i++)
newList[i] = list[list.length - 1 - i];
list = newList;
}
}
}
(b)
(a)
7.18
Where are the arrays stored during execution? Show the contents of the stack and
heap during and after executing displayArray, countLetters, displayCounts
in Listing 7.4.
7.9 Variable-Length Argument Lists
Key
Point
A variable number of arguments of the same type can be passed to a method and
treated as an array.
You can pass a variable number of arguments of the same type to a method. The parameter in
the method is declared as follows:
typeName... parameterName
In the method declaration, you specify the type followed by an ellipsis (...). Only one
variable-length parameter may be specified in a method, and this parameter must be the last
parameter. Any regular parameters must precede it.
7.10 Searching Arrays 265
Java treats a variable-length parameter as an array. You can pass an array or a variable
number of arguments to a variable-length parameter. When invoking a method with a variable number of arguments, Java creates an array and passes the arguments to it. Listing 7.5
contains a method that prints the maximum value in a list of an unspecified number of values.
LISTING 7.5 VarArgsDemo.java
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
public class VarArgsDemo {
public static void main(String[] args) {
printMax(34, 3, 3, 2, 56.5);
printMax(new double[]{1, 2, 3});
}
public static void printMax(double... numbers) {
if (numbers.length == 0) {
System.out.println("No argument passed");
return;
}
pass variable-length arg list
pass an array arg
a variable-length arg
parameter
double result = numbers[0];
for (int i = 1; i < numbers.length; i++)
if (numbers[i] > result)
result = numbers[i];
System.out.println("The max value is " + result);
}
}
Line 3 invokes the printMax method with a variable-length argument list passed to the array
numbers. If no arguments are passed, the length of the array is 0 (line 8).
Line 4 invokes the printMax method with an array.
7.19
What is wrong in the following method header?
public static void print(String... strings, double... numbers)
public static void print(double... numbers, String name)
public static double... print(double d1, double d2)
7.20
✓
Check
Point
Can you invoke the printMax method in Listing 7.5 using the following statements?
printMax(1, 2, 2, 1, 4);
printMax(new double[]{1, 2, 3});
printMax(new int[]{1, 2, 3});
7.10 Searching Arrays
If an array is sorted, binary search is more efficient than linear search for finding an
element in the array.
Searching is the process of looking for a specific element in an array—for example, discovering whether a certain score is included in a list of scores. Searching is a common task in
computer programming. Many algorithms and data structures are devoted to searching. This
section discusses two commonly used approaches, linear search and binary search.
7.10.1
The Linear Search Approach
The linear search approach compares the key element key sequentially with each element in
the array. It continues to do so until the key matches an element in the array or the array is
exhausted without a match being found. If a match is made, the linear search returns the index
Key
Point
linear search
binary search
266 Chapter 7
Single-Dimensional Arrays
of the element in the array that matches the key. If no match is found, the search returns -1.
The linearSearch method in Listing 7.6 gives the solution.
linear search animation on
Companion Website
LISTING 7.6
LinearSearch.java
1 public class LinearSearch {
2
/** The method for finding a key in the list */
3
public static int linearSearch(int[] list, int key) {
4
for (int i = 0; i < list.length; i++) {
5
if (key == list[i])
[0] [1] [2] …
6
return i;
7
}
list
8
return -1;
key Compare key with list[i] for i = 0, 1, …
9
}
10 }
To better understand this method, trace it with the following statements:
1
2
3
4
int[]
int i
int j
int k
list = {1, 4, 4, 2, 5, -3, 6, 2};
= linearSearch(list, 4); // Returns 1
= linearSearch(list, -4); // Returns -1
= linearSearch(list, -3); // Returns 5
The linear search method compares the key with each element in the array. The elements can
be in any order. On average, the algorithm will have to examine half of the elements in an
array before finding the key, if it exists. Since the execution time of a linear search increases
linearly as the number of array elements increases, linear search is inefficient for a large array.
7.10.2
The Binary Search Approach
Binary search is the other common search approach for a list of values. For binary search to
work, the elements in the array must already be ordered. Assume that the array is in ascending
order. The binary search first compares the key with the element in the middle of the array.
Consider the following three cases:
binary search animation on
Companion Website
■
If the key is less than the middle element, you need to continue to search for the key
only in the first half of the array.
■
If the key is equal to the middle element, the search ends with a match.
■
If the key is greater than the middle element, you need to continue to search for the
key only in the second half of the array.
Clearly, the binary search method eliminates at least half of the array after each comparison.
Sometimes you eliminate half of the elements, and sometimes you eliminate half plus one.
Suppose that the array has n elements. For convenience, let n be a power of 2. After the first
comparison, n/2 elements are left for further search; after the second comparison, (n/2)/2
elements are left. After the kth comparison, n/2k elements are left for further search. When
k = log2n, only one element is left in the array, and you need only one more comparison.
Therefore, in the worst case when using the binary search approach, you need log2n+1 comparisons to find an element in the sorted array. In the worst case for a list of 1024 (210) elements, binary search requires only 11 comparisons, whereas a linear search requires 1023
comparisons in the worst case.
The portion of the array being searched shrinks by half after each comparison. Let low and
high denote, respectively, the first index and last index of the array that is currently being
searched. Initially, low is 0 and high is list.length–1. Let mid denote the index of the
middle element, so mid is (low + high)/2. Figure 7.9 shows how to find key 11 in the list
{2, 4, 7, 10, 11, 45, 50, 59, 60, 66, 69, 70, 79} using binary search.
7.10 Searching Arrays 267
You now know how the binary search works. The next task is to implement it in Java.
Don’t rush to give a complete implementation. Implement it incrementally, one step at a time.
You may start with the first iteration of the search, as shown in Figure 7.10a. It compares the
key with the middle element in the list whose low index is 0 and high index is list.length
- 1. If key < list[mid], set the high index to mid - 1; if key == list[mid], a match
is found and return mid; if key > list[mid], set the low index to mid + 1.
Next consider implementing the method to perform the search repeatedly by adding a loop,
as shown in Figure 7.10b. The search ends if the key is found, or if the key is not found when
low > high.
When the key is not found, low is the insertion point where a key would be inserted to
maintain the order of the list. It is more useful to return the insertion point than -1. The
method must return a negative value to indicate that the key is not in the list. Can it simply
return –low? No. If the key is less than list[0], low would be 0. -0 is 0. This would indicate that the key matches list[0]. A good choice is to let the method return –low – 1 if the
key is not in the list. Returning –low – 1 indicates not only that the key is not in the list, but
also where the key would be inserted.
low
key is 11
key 50
mid
high
[0] [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
list
2
4
low
7
10 11 45 50 59 60 66 69 70 79
mid
high
[0] [1] [2] [3] [4] [5]
key 7
list
2
4
7
10 11 45
low mid high
[3] [4] [5]
key 11
list
10 11 45
FIGURE 7.9 Binary search eliminates half of the list from further consideration after each
comparison.
public static int binarySearch(
int[] list, int key) {
int low = 0;
int high = list.length - 1;
public static int binarySearch(
int[] list, int key) {
int low = 0;
int high = list.length - 1;
int mid = (low + high) / 2;
if (key < list[mid])
high = mid - 1;
else if (key == list[mid])
return mid;
else
low = mid + 1;
while (high >= low) {
int mid = (low + high) / 2;
if (key < list[mid])
high = mid - 1;
else if (key == list[mid])
return mid;
else
low = mid + 1;
}
return -1; // Not found
}
}
(a) Version 1
FIGURE 7.10 Binary search is implemented incrementally.
(b) Version 2
why not -1?
268 Chapter 7
Single-Dimensional Arrays
The complete program is given in Listing 7.7.
LISTING 7.7
first half
second half
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
BinarySearch.java
public class BinarySearch {
/** Use binary search to find the key in the list */
public static int binarySearch(int[] list, int key) {
int low = 0;
int high = list.length - 1;
while (high >= low) {
int mid = (low + high) / 2;
if (key < list[mid])
high = mid - 1;
else if (key == list[mid])
return mid;
else
low = mid + 1;
}
return –low - 1; // Now high < low, key not found
}
}
The binary search returns the index of the search key if it is contained in the list (line 12).
Otherwise, it returns –low – 1 (line 17).
What would happen if we replaced (high >= low) in line 7 with (high > low)? The
search would miss a possible matching element. Consider a list with just one element. The
search would miss the element.
Does the method still work if there are duplicate elements in the list? Yes, as long as the
elements are sorted in increasing order. The method returns the index of one of the matching
elements if the element is in the list.
To better understand this method, trace it with the following statements and identify low
and high when the method returns.
int[]
int i
int j
int k
int l
int m
list = {2, 4, 7, 10, 11, 45, 50, 59, 60, 66, 69, 70, 79};
= BinarySearch.binarySearch(list, 2); // Returns 0
= BinarySearch.binarySearch(list, 11); // Returns 4
= BinarySearch.binarySearch(list, 12); // Returns –6
= BinarySearch.binarySearch(list, 1); // Returns –1
= BinarySearch.binarySearch(list, 3); // Returns –2
Here is the table that lists the low and high values when the method exits and the value
returned from invoking the method.
Low
High
Value Returned
binarySearch(list, 2)
Method
0
1
0
binarySearch(list, 11)
3
5
4
binarySearch(list, 12)
5
4
-6
binarySearch(list, 1)
0
-1
-1
binarySearch(list, 3)
1
0
-2
Note
binary search benefits
Linear search is useful for finding an element in a small array or an unsorted array, but
it is inefficient for large arrays. Binary search is more efficient, but it requires that the
array be presorted.
7.11 Sorting Arrays 269
7.21
If high is a very large integer such as the maximum int value 2147483647, (low
+ high) / 2 may cause overflow. How do you fix it to avoid overflow?
7.22
Use Figure 7.9 as an example to show how to apply the binary search approach to a
search for key 10 and key 12 in list {2, 4, 7, 10, 11, 45, 50, 59, 60, 66, 69, 70, 79}.
If the binary search method returns -4, is the key in the list? Where should the key
be inserted if you wish to insert the key into the list?
7.23
✓
Check
Point
7.11 Sorting Arrays
Sorting, like searching, is a common task in computer programming. Many different
algorithms have been developed for sorting. This section introduces an intuitive sorting algorithm: selection sort.
Suppose that you want to sort a list in ascending order. Selection sort finds the smallest
number in the list and swaps it with the first element. It then finds the smallest number
remaining and swaps it with the second element, and so on, until only a single number
remains. Figure 7.11 shows how to sort the list {2, 9, 5, 4, 8, 1, 6} using selection sort.
Key
Point
VideoNote
Selection sort
selection sort
swap
Select 1 (the smallest) and swap it
with 2 (the first) in the list.
2
9
5
4
1
6
8
2
6
Select 2 (the smallest) and swap it
with 9 (the first) in the remaining
list.
8
swap
The number 1 is now in the
correct position and thus no
longer needs to be considered.
1
9
5
4
swap
The number 2 is now in the
correct position and thus no
longer needs to be considered.
1
2
5
4
8
9
6
Select 4 (the smallest) and swap it
with 5 (the first) in the remaining
list.
The number 4 is now in the
correct position and thus no
longer needs to be considered.
1
2
4
5
8
9
6
5 is the smallest and in the right
position. No swap is necessary.
6
Select 6 (the smallest) and swap it
with 8 (the first) in the remaining
list.
swap
The number 5 is now in the
correct position and thus no
longer needs to be considered.
1
2
4
5
8
9
swap
The number 6 is now in the
correct position and thus no
longer needs to be considered.
1
2
4
5
6
9
8
Select 8 (the smallest) and swap it
with 9 (the first) in the remaining
list.
The number 8 is now in the
correct position and thus no
longer needs to be considered.
1
2
4
5
6
8
9
Since there is only one element
remaining in the list, the sort is
completed.
FIGURE 7.11 Selection sort repeatedly selects the smallest number and swaps it with the first number in the list.
You know how the selection-sort approach works. The task now is to implement it in Java.
Beginners find it difficult to develop a complete solution on the first attempt. Start by writing
the code for the first iteration to find the smallest element in the list and swap it with the first
element, and then observe what would be different for the second iteration, the third, and so
on. The insight this gives will enable you to write a loop that generalizes all the iterations.
selection sort animation on
Companion Website
270 Chapter 7
Single-Dimensional Arrays
The solution can be described as follows:
for (int i = 0; i < list.length - 1; i++) {
select the smallest element in list[i..list.length-1];
swap the smallest with list[i], if necessary;
// list[i] is in its correct position.
// The next iteration applies on list[i+1..list.length-1]
}
Listing 7.8 implements the solution.
LISTING 7.8 SelectionSort.java
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
select
swap
public class SelectionSort {
/** The method for sorting the numbers */
public static void selectionSort(double[] list) {
for (int i = 0; i < list.length - 1; i++) {
// Find the minimum in the list[i..list.length-1]
double currentMin = list[i];
int currentMinIndex = i;
for (int j = i + 1; j < list.length; j++) {
if (currentMin > list[j]) {
currentMin = list[j];
currentMinIndex = j;
}
}
// Swap list[i] with list[currentMinIndex] if necessary
if (currentMinIndex != i) {
list[currentMinIndex] = list[i];
list[i] = currentMin;
}
}
}
}
The selectionSort(double[] list) method sorts any array of double elements. The
method is implemented with a nested for loop. The outer loop (with the loop control variable i) (line 4) is iterated in order to find the smallest element in the list, which ranges from
list[i] to list[list.length-1], and exchange it with list[i].
The variable i is initially 0. After each iteration of the outer loop, list[i] is in the right
place. Eventually, all the elements are put in the right place; therefore, the whole list is sorted.
To understand this method better, trace it with the following statements:
double[] list = {1, 9, 4.5, 6.6, 5.7, -4.5};
SelectionSort.selectionSort(list);
✓
Check
Point
7.24
7.25
Use Figure 7.11 as an example to show how to apply the selection-sort approach to
sort {3.4, 5, 3, 3.5, 2.2, 1.9, 2}.
How do you modify the selectionSort method in Listing 7.8 to sort numbers in
decreasing order?
7.12 The Arrays Class
Key
Point
The java.util.Arrays class contains useful methods for common array operations
such as sorting and searching.
7.12 The Arrays Class 271
The java.util.Arrays class contains various static methods for sorting and searching
arrays, comparing arrays, filling array elements, and returning a string representation of the
array. These methods are overloaded for all primitive types.
You can use the sort or parallelSort method to sort a whole array or a partial array.
For example, the following code sorts an array of numbers and an array of characters.
sort
parallelSort
double[] numbers = {6.0, 4.4, 1.9, 2.9, 3.4, 3.5};
java.util.Arrays.sort(numbers); // Sort the whole array
java.util.Arrays.parallelSort(numbers); // Sort the whole array
char[] chars = {'a', 'A', '4', 'F', 'D', 'P'};
java.util.Arrays.sort(chars, 1, 3); // Sort part of the array
java.util.Arrays.parallelSort(chars, 1, 3); // Sort part of the array
Invoking sort(numbers) sorts the whole array numbers. Invoking sort(chars, 1, 3)
sorts a partial array from chars[1] to chars[3-1]. parallelSort is more efficient if
your computer has multiple processors.
You can use the binarySearch method to search for a key in an array. The array must be presorted in increasing order. If the key is not in the array, the method returns –(insertionIndex
+ 1). For example, the following code searches the keys in an array of integers and an array
of characters.
binarySearch
int[] list = {2, 4, 7, 10, 11, 45, 50, 59, 60, 66, 69, 70, 79};
System.out.println("1. Index is " +
java.util.Arrays.binarySearch(list, 11));
System.out.println("2. Index is " +
java.util.Arrays.binarySearch(list, 12));
char[] chars = {'a', 'c', 'g', 'x', 'y', 'z'};
System.out.println("3. Index is " +
java.util.Arrays.binarySearch(chars, 'a'));
System.out.println("4. Index is " +
java.util.Arrays.binarySearch(chars, 't'));
The output of the preceding code is
1. Index is 4
2. Index is -6
3. Index is 0
4. Index is -4
You can use the equals method to check whether two arrays are strictly equal. Two arrays
are strictly equal if their corresponding elements are the same. In the following code, list1
and list2 are equal, but list2 and list3 are not.
equals
int[] list1 = {2, 4, 7, 10};
int[] list2 = {2, 4, 7, 10};
int[] list3 = {4, 2, 7, 10};
System.out.println(java.util.Arrays.equals(list1, list2)); // true
System.out.println(java.util.Arrays.equals(list2, list3)); // false
You can use the fill method to fill in all or part of the array. For example, the following code
fills list1 with 5 and fills 8 into elements list2[1] through list2[5-1].
int[] list1 = {2, 4, 7, 10};
int[] list2 = {2, 4, 7, 7, 7, 10};
java.util.Arrays.fill(list1, 5); // Fill 5 to the whole array
java.util.Arrays.fill(list2, 1, 5, 8); // Fill 8 to a partial array
fill
272 Chapter 7
Single-Dimensional Arrays
You can also use the toString method to return a string that represents all elements in the array.
This is a quick and simple way to display all elements in the array. For example, the following code
toString
int[] list = {2, 4, 7, 10};
System.out.println(Arrays.toString(list));
displays [2, 4, 7, 10].
✓
Check
Point
7.26
7.27
7.28
What types of array can be sorted using the java.util.Arrays.sort method?
Does this sort method create a new array?
To apply java.util.Arrays.binarySearch(array, key), should the array
be sorted in increasing order, in decreasing order, or neither?
Show the output of the following code:
int[] list1 = {2, 4, 7, 10};
java.util.Arrays.fill(list1, 7);
System.out.println(java.util.Arrays.toString(list1));
int[] list2 = {2, 4, 7, 10};
System.out.println(java.util.Arrays.toString(list2));
System.out.print(java.util.Arrays.equals(list1, list2));
7.13 Command-Line Arguments
Key
Point
VideoNote
Command-line arguments
The main method can receive string arguments from the command line.
Perhaps you have already noticed the unusual header for the main method, which has the
parameter args of String[] type. It is clear that args is an array of strings. The main
method is just like a regular method with a parameter. You can call a regular method by passing actual parameters. Can you pass arguments to main? Yes, of course you can. In the following examples, the main method in class TestMain is invoked by a method in A.
public class A {
public static void main(String[] args) {
String[] strings = {"New York",
"Boston", "Atlanta"};
TestMain.main(strings);
}
}
public class TestMain {
public static void main(String[] args) {
for (int i = 0; i < args.length; i++)
System.out.println(args[i]);
}
}
A main method is just a regular method. Furthermore, you can pass arguments from the
command line.
7.13.1
Passing Strings to the main Method
You can pass strings to a main method from the command line when you run the program.
The following command line, for example, starts the program TestMain with three strings:
arg0, arg1, and arg2:
java TestMain arg0 arg1 arg2
arg0, arg1, and arg2 are strings, but they don’t have to appear in double quotes on the
command line. The strings are separated by a space. A string that contains a space must be
enclosed in double quotes. Consider the following command line:
java TestMain "First num" alpha 53
7.13 Command-Line Arguments 273
It starts the program with three strings: First num, alpha, and 53. Since First num is a
string, it is enclosed in double quotes. Note that 53 is actually treated as a string. You can use
"53" instead of 53 in the command line.
When the main method is invoked, the Java interpreter creates an array to hold the command-line arguments and pass the array reference to args. For example, if you invoke a
program with n arguments, the Java interpreter creates an array like this one:
args = new String[n];
The Java interpreter then passes args to invoke the main method.
Note
If you run the program with no strings passed, the array is created with new String[0].
In this case, the array is empty with length 0. args references to this empty array.
Therefore, args is not null, but args.length is 0.
7.13.2
Case Study: Calculator
Suppose you are to develop a program that performs arithmetic operations on integers. The
program receives an expression in one string argument. The expression consists of an integer followed by an operator and another integer. For example, to add two integers, use this
command:
java Calculator 2 + 3
The program will display the following output:
2 + 3 = 5
Figure 7.12 shows sample runs of the program.
The strings passed to the main program are stored in args, which is an array of strings. The
first string is stored in args[0], and args.length is the number of strings passed.
Here are the steps in the program:
1. Use args.length to determine whether the expression has been provided as three
arguments in the command line. If not, terminate the program using System.exit(1).
2. Perform a binary arithmetic operation on the operands args[0] and args[2] using the
operator in args[1].
Add
Subtract
Multiply
Divide
FIGURE 7.12 The program takes three arguments (operand1 operator operand2) from
the command line and displays the expression and the result of the arithmetic operation.
VideoNote
Command-line argument
274 Chapter 7
Single-Dimensional Arrays
The program is shown in Listing 7.9.
LISTING 7.9
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
check argument
check operator
Calculator.java
public class Calculator {
/** Main method */
public static void main(String[] args) {
// Check number of strings passed
if (args.length != 3) {
System.out.println(
"Usage: java Calculator operand1 operator operand2");
System.exit(0);
}
// The result of the operation
int result = 0;
// Determine the operator
switch (args[1].charAt(0)) {
case '+': result = Integer.parseInt(args[0]) +
Integer.parseInt(args[2]);
break;
case '-': result = Integer.parseInt(args[0]) Integer.parseInt(args[2]);
break;
case '.': result = Integer.parseInt(args[0]) *
Integer.parseInt(args[2]);
break;
case '/': result = Integer.parseInt(args[0]) /
Integer.parseInt(args[2]);
}
// Display result
System.out.println(args[0] + ' ' + args[1] + ' ' + args[2]
+ " = " + result);
}
}
Integer.parseInt(args[0]) (line 16) converts a digital string into an integer. The string
must consist of digits. If not, the program will terminate abnormally.
We used the . symbol for multiplication, not the common * symbol. The reason for this is
that the * symbol refers to all the files in the current directory when it is used on a command
line. The following program displays all the files in the current directory when issuing the
command java Test *:
public class Test {
public static void main(String[] args) {
for (int i = 0; i < args.length; i++)
System.out.println(args[i]);
}
}
To circumvent this problem, we will have to use a different symbol for the multiplication operator.
✓
Check
Point
7.29
This book declares the main method as
public static void main(String[] args)
Can it be replaced by one of the following lines?
public static void main(String args[])
public static void main(String[] x)
Chapter Summary 275
public static void main(String x[])
static void main(String x[])
7.30
Show the output of the following program when invoked using
1. java Test I have a dream
2. java Test “1 2 3”
3. java Test
public class Test {
public static void main(String[] args) {
System.out.println("Number of strings is " + args.length);
for (int i = 0; i < args.length; i++)
System.out.println(args[i]);
}
}
KEY TERMS
anonymous array 258
array 246
array initializer 248
binary search 265
garbage collection 256
index 246
indexed variable 248
linear search 265
off-by-one error 251
selection sort 269
CHAPTER SUMMARY
1. A variable is declared as an array type using the syntax elementType[]
arrayRefVar
or elementType arrayRefVar[]. The style elementType[] arrayRefVar is
preferred, although elementType arrayRefVar[] is legal.
2. Unlike declarations for primitive data type variables, the declaration of an array variable
does not allocate any space in memory for the array. An array variable is not a primitive
data type variable. An array variable contains a reference to an array.
3. You cannot assign elements to an array unless it has already been created. You
can create an array by using the new operator with the following syntax: new
elementType[arraySize].
4. Each element in the array is represented using the syntax arrayRefVar[index]. An
index must be an integer or an integer expression.
5. After an array is created, its size becomes permanent and can be obtained using
arrayRefVar.length. Since the index of an array always begins with 0, the last
index is always arrayRefVar.length - 1. An out-of-bounds error will occur if you
attempt to reference elements beyond the bounds of an array.
6. Programmers often mistakenly reference the first element in an array with index 1, but
it should be 0. This is called the index off-by-one error.
276 Chapter 7
Single-Dimensional Arrays
7. When an array is created, its elements are assigned the default value of 0 for the numeric
primitive data types, \u0000 for char types, and false for boolean types.
8. Java has a shorthand notation, known as the array initializer, which combines declaring
an array, creating an array, and initializing an array in one statement, using the syntax
elementType[] arrayRefVar = {value0, value1, ..., valuek}.
9. When you pass an array argument to a method, you are actually passing the reference
of the array; that is, the called method can modify the elements in the caller’s original
array.
10. If an array is sorted, binary search is more efficient than linear search for finding an
element in the array.
11. Selection sort finds the smallest number in the list and swaps it with the first element.
It then finds the smallest number remaining and swaps it with the first element in the
remaining list, and so on, until only a single number remains.
QUIZ
Answer the quiz for this chapter online at www.cs.armstrong.edu/liang/intro10e/quiz.html.
PROGRAMMING EXERCISES
Sections 7.2–7.5
*7.1 (Assign grades) Write a program that reads student scores, gets the best score,
and then assigns grades based on the following scheme:
Grade is A if score is Ú best - 10
Grade is B if score is Ú best - 20;
Grade is C if score is Ú best - 30;
Grade is D if score is Ú best - 40;
Grade is F otherwise.
The program prompts the user to enter the total number of students, then prompts
the user to enter all of the scores, and concludes by displaying the grades. Here
is a sample run:
Enter the number of students: 4
Enter 4
Student
Student
Student
Student
scores:
0 score
1 score
2 score
3 score
40
is
is
is
is
55
40
55
70
58
70 58
and grade
and grade
and grade
and grade
is
is
is
is
C
B
A
B
7.2 (Reverse the numbers entered) Write a program that reads ten integers and displays them in the reverse of the order in which they were read.
Programming Exercises 277
**7.3 (Count occurrence of numbers) Write a program that reads the integers between 1
and 100 and counts the occurrences of each. Assume the input ends with 0. Here
is a sample run of the program:
Enter the integers between 1 and 100: 2 5 6 5 4 3 23 43 2 0
2 occurs 2 times
3 occurs 1 time
4 occurs 1 time
5 occurs 2 times
6 occurs 1 time
23 occurs 1 time
43 occurs 1 time
Note that if a number occurs more than one time, the plural word “times” is used
in the output.
7.4 (Analyze scores) Write a program that reads an unspecified number of scores and
**7.5
determines how many scores are above or equal to the average and how many
scores are below the average. Enter a negative number to signify the end of the
input. Assume that the maximum number of scores is 100.
(Print distinct numbers) Write a program that reads in ten numbers and displays
the number of distinct numbers and the distinct numbers separated by exactly one
space (i.e., if a number appears multiple times, it is displayed only once). (Hint:
Read a number and store it to an array if it is new. If the number is already in the
array, ignore it.) After the input, the array contains the distinct numbers. Here is
the sample run of the program:
Enter ten numbers: 1 2 3 2 1 6 3 4 5 2
The number of distinct number is 6
The distinct numbers are: 1 2 3 6 4 5
*7.6 (Revise Listing 5.15, PrimeNumber.java) Listing 5.15 determines whether a num-
*7.7
ber n is prime by checking whether 2, 3, 4, 5, 6, . . . , n/2 is a divisor. If a divisor
is found, n is not prime. A more efficient approach is to check whether any of the
prime numbers less than or equal to 2n can divide n evenly. If not, n is prime.
Rewrite Listing 5.15 to display the first 50 prime numbers using this approach.
You need to use an array to store the prime numbers and later use them to check
whether they are possible divisors for n.
(Count single digits) Write a program that generates 100 random integers between
0 and 9 and displays the count for each number. (Hint: Use an array of ten integers,
say counts, to store the counts for the number of 0s, 1s, . . . , 9s.)
Sections 7.6–7.8
7.8 (Average an array) Write two overloaded methods that return the average of an
array with the following headers:
public static int average(int[] array)
public static double average(double[] array)
Write a test program that prompts the user to enter ten double values, invokes this
method, and displays the average value.
278 Chapter 7
Single-Dimensional Arrays
7.9 (Find the smallest element) Write a method that finds the smallest element in an
array of double values using the following header:
public static double min(double[] array)
Write a test program that prompts the user to enter ten numbers, invokes this
method to return the minimum value, and displays the minimum value. Here is a
sample run of the program:
Enter ten numbers: 1.9 2.5 3.7 2 1.5 6 3 4 5 2
The minimum number is: 1.5
7.10 (Find the index of the smallest element) Write a method that returns the index of
the smallest element in an array of integers. If the number of such elements is
greater than 1, return the smallest index. Use the following header:
public static int indexOfSmallestElement(double[] array)
*7.11
Write a test program that prompts the user to enter ten numbers, invokes this
method to return the index of the smallest element, and displays the index.
(Statistics: compute deviation) Programming Exercise 5.45 computes the standard deviation of numbers. This exercise uses a different but equivalent formula to
compute the standard deviation of n numbers.
n
mean =
a xi
i=1
n
n
=
x1 + x2 + g + xn
n
deviation =
2
a (xi - mean)
i=1
H
n - 1
To compute the standard deviation with this formula, you have to store the individual numbers using an array, so that they can be used after the mean is obtained.
Your program should contain the following methods:
/** Compute the deviation of double values */
public static double deviation(double[] x)
/** Compute the mean of an array of double values */
public static double mean(double[] x)
Write a test program that prompts the user to enter ten numbers and displays the
mean and standard deviation, as shown in the following sample run:
Enter ten numbers: 1.9 2.5 3.7 2 1 6 3 4 5 2
The mean is 3.11
The standard deviation is 1.55738
*7.12 (Reverse an array) The
reverse method in Section 7.7 reverses an array by
copying it to a new array. Rewrite the method that reverses the array passed in
the argument and returns this array. Write a test program that prompts the user to
Programming Exercises 279
enter ten numbers, invokes the method to reverse the numbers, and displays the
numbers.
Section 7.9
*7.13 (Random number chooser) Write a method that returns a random number between
1 and 54, excluding the numbers passed in the argument. The method header is
specified as follows:
public static int getRandom(int... numbers)
7.14 (Computing gcd) Write a method that returns the gcd of an unspecified number
of integers. The method header is specified as follows:
public static int gcd(int... numbers)
Write a test program that prompts the user to enter five numbers, invokes the
method to find the gcd of these numbers, and displays the gcd.
Sections 7.10–7.12
7.15 (Eliminate duplicates) Write a method that returns a new array by eliminating the
duplicate values in the array using the following method header:
public static int[] eliminateDuplicates(int[] list)
Write a test program that reads in ten integers, invokes the method, and displays
the result. Here is the sample run of the program:
Enter ten numbers: 1 2 3 2 1 6 3 4 5 2
The distinct numbers are: 1 2 3 6 4 5
7.16 (Execution time) Write a program that randomly generates an array of 100,000
integers and a key. Estimate the execution time of invoking the linearSearch
method in Listing 7.6. Sort the array and estimate the execution time of invoking the binarySearch method in Listing 7.7. You can use the following code
template to obtain the execution time:
long startTime = System.currentTimeMillis();
perform the task;
long endTime = System.currentTimeMillis();
long executionTime = endTime - startTime;
**7.17 (Sort students) Write a program that prompts the user to enter the number of stu**7.18
dents, the students’ names, and their scores, and prints student names in decreasing order of their scores.
(Bubble sort) Write a sort method that uses the bubble-sort algorithm. The bubblesort algorithm makes several passes through the array. On each pass, successive
neighboring pairs are compared. If a pair is not in order, its values are swapped;
otherwise, the values remain unchanged. The technique is called a bubble sort or
sinking sort because the smaller values gradually “bubble” their way to the top
and the larger values “sink” to the bottom. Write a test program that reads in ten
double numbers, invokes the method, and displays the sorted numbers.
280 Chapter 7
Single-Dimensional Arrays
**7.19 (Sorted?) Write the following method that returns true if the list is already sorted
in increasing order.
public static boolean isSorted(int[] list)
Write a test program that prompts the user to enter a list and displays whether
the list is sorted or not. Here is a sample run. Note that the first number in the
input indicates the number of the elements in the list. This number is not part
of the list.
Enter list: 8 10 1 5 16 61 9 11 1
The list is not sorted
Enter list: 10 1 1 3 4 4 5 7 9 11 21
The list is already sorted
*7.20 (Revise selection sort) In Section 7.11, you used selection sort to sort an array.
***7.21
The selection-sort method repeatedly finds the smallest number in the current
array and swaps it with the first. Rewrite this program by finding the largest number and swapping it with the last. Write a test program that reads in ten double
numbers, invokes the method, and displays the sorted numbers.
(Game: bean machine) The bean machine, also known as a quincunx or the Galton box, is a device for statistics experiments named after English scientist Sir
Francis Galton. It consists of an upright board with evenly spaced nails (or pegs)
in a triangular form, as shown in Figure 7.13.
(a)
(b)
(c)
FIGURE 7.13 Each ball takes a random path and falls into a slot.
Balls are dropped from the opening of the board. Every time a ball hits a nail, it
has a 50% chance of falling to the left or to the right. The piles of balls are accumulated in the slots at the bottom of the board.
Write a program that simulates the bean machine. Your program should prompt
the user to enter the number of the balls and the number of the slots in the machine.
Simulate the falling of each ball by printing its path. For example, the path for
the ball in Figure 7.13b is LLRRLLR and the path for the ball in Figure 7.13c is
Programming Exercises 281
RLRRLRR. Display the final buildup of the balls in the slots in a histogram. Here
is a sample run of the program:
Enter the number of balls to drop: 5
Enter the number of slots in the bean machine: 8
LRLRLRR
RRLLLRR
LLRLLRR
RRLLLLL
LRLRRLR
O
O
OOO
(Hint: Create an array named slots. Each element in slots stores the number of balls in a slot. Each ball falls into a slot via a path. The number of Rs in
a path is the position of the slot where the ball falls. For example, for the path
LRLRLRR, the ball falls into slots[4], and for the path is RRLLLLL, the ball
falls into slots[2].)
***7.22 (Game: Eight Queens) The classic Eight Queens puzzle is to place eight queens
on a chessboard such that no two queens can attack each other (i.e., no two queens
are on the same row, same column, or same diagonal). There are many possible
solutions. Write a program that displays one such solution. A sample output is
shown below:
|Q| | | | | | | |
| | | | |Q| | | |
| | | | | | | |Q|
| | | | | |Q| | |
| | |Q| | | | | |
| | | | | | |Q| |
| |Q| | | | | | |
| | | |Q| | | | |
**7.23 (Game: locker puzzle) A school has 100 lockers and 100 students. All lockers are
**7.24
closed on the first day of school. As the students enter, the first student, denoted
S1, opens every locker. Then the second student, S2, begins with the second
locker, denoted L2, and closes every other locker. Student S3 begins with the
third locker and changes every third locker (closes it if it was open, and opens it if
it was closed). Student S4 begins with locker L4 and changes every fourth locker.
Student S5 starts with L5 and changes every fifth locker, and so on, until student
S100 changes L100.
After all the students have passed through the building and changed the lockers,
which lockers are open? Write a program to find your answer and display all
open locker numbers separated by exactly one space.
(Hint: Use an array of 100 Boolean elements, each of which indicates whether a
locker is open (true) or closed (false). Initially, all lockers are closed.)
(Simulation: coupon collector’s problem) Coupon collector is a classic statistics
problem with many practical applications. The problem is to pick objects from
a set of objects repeatedly and find out how many picks are needed for all the
VideoNote
Coupon collector’s problem
282 Chapter 7
Single-Dimensional Arrays
objects to be picked at least once. A variation of the problem is to pick cards from
a shuffled deck of 52 cards repeatedly and find out how many picks are needed
before you see one of each suit. Assume a picked card is placed back in the deck
before picking another. Write a program to simulate the number of picks needed
to get four cards from each suit and display the four cards picked (it is possible a
card may be picked twice). Here is a sample run of the program:
Queen of Spades
5 of Clubs
Queen of Hearts
4 of Diamonds
Number of picks: 12
7.25 (Algebra: solve quadratic equations) Write a method for solving a quadratic
equation using the following header:
public static int solveQuadratic(double[] eqn, double[] roots)
7.26
The coefficients of a quadratic equation ax2 + bx + c = 0 are passed to the
array eqn and the real roots are stored in roots. The method returns the number of real roots. See Programming Exercise 3.1 on how to solve a quadratic
equation.
Write a program that prompts the user to enter values for a, b, and c and displays
the number of real roots and all real roots.
(Strictly identical arrays) The arrays list1 and list2 are strictly identical
if their corresponding elements are equal. Write a method that returns true if
list1 and list2 are strictly identical, using the following header:
public static boolean equals(int[] list1, int[] list2)
Write a test program that prompts the user to enter two lists of integers and displays whether the two are strictly identical. Here are the sample runs. Note that
the first number in the input indicates the number of the elements in the list. This
number is not part of the list.
Enter list1: 5 2 5 6 1 6
Enter list2: 5 2 5 6 1 6
Two lists are strictly identical
Enter list1: 5 2 5 6 6 1
Enter list2: 5 2 5 6 1 6
Two lists are not strictly identical
7.27 (Identical arrays) The arrays
list1 and list2 are identical if they have the
same contents. Write a method that returns true if list1 and list2 are identical, using the following header:
public static boolean equals(int[] list1, int[] list2)
Programming Exercises 283
Write a test program that prompts the user to enter two lists of integers and displays whether the two are identical. Here are the sample runs. Note that the first
number in the input indicates the number of the elements in the list. This number
is not part of the list.
Enter list1: 5 2 5 6 6 1
Enter list2: 5 5 2 6 1 6
Two lists are identical
Enter list1: 5 5 5 6 6 1
Enter list2: 5 2 5 6 1 6
Two lists are not identical
*7.28 (Math: combinations) Write a program that prompts the user to enter 10 integers
and displays all combinations of picking two numbers from the 10.
*7.29 (Game: pick four cards) Write a program that picks four cards from a deck of 52
*7.30
cards and computes their sum. An Ace, King, Queen, and Jack represent 1, 13,
12, and 11, respectively. Your program should display the number of picks that
yields the sum of 24.
(Pattern recognition: consecutive four equal numbers) Write the following
method that tests whether the array has four consecutive numbers with the same
value.
public static boolean isConsecutiveFour(int[] values)
Write a test program that prompts the user to enter a series of integers and displays if the series contains four consecutive numbers with the same value. Your
program should first prompt the user to enter the input size—i.e., the number of
values in the series. Here are sample runs:
Enter the number of values: 8
Enter the values: 3 4 5 5 5 5 4 5
The list has consecutive fours
Enter the number of values: 9
Enter the values: 3 4 5 5 6 5 5 4 5
The list has no consecutive fours
**7.31 (Merge two sorted lists) Write the following method that merges two sorted lists
into a new sorted list.
public static int[] merge(int[] list1, int[] list2)
VideoNote
Consecutive four
284 Chapter 7
Single-Dimensional Arrays
Implement the method in a way that takes at most list1.length + list2.
length comparisons. Write a test program that prompts the user to enter two
sorted lists and displays the merged list. Here is a sample run. Note that the first
number in the input indicates the number of the elements in the list. This number
is not part of the list.
Enter list1: 5 1 5 16 61 111
Enter list2: 4 2 4 5 6
The merged list is 1 2 4 5 5 6 16 61 111
**7.32 (Partition of a list) Write the following method that partitions the list using the
first element, called a pivot.
public static int partition(int[] list)
After the partition, the elements in the list are rearranged so that all the elements
before the pivot are less than or equal to the pivot and the elements after the pivot
are greater than the pivot. The method returns the index where the pivot is located
in the new list. For example, suppose the list is {5, 2, 9, 3, 6, 8}. After the partition, the list becomes {3, 2, 5, 9, 6, 8}. Implement the method in a way that takes
at most list.length comparisons. Write a test program that prompts the user
to enter a list and displays the list after the partition. Here is a sample run. Note
that the first number in the input indicates the number of the elements in the list.
This number is not part of the list.
Enter list: 8 10 1 5 16 61 9 11 1
After the partition, the list is 9 1 5 1 10 61 11 16
*7.33 (Culture: Chinese Zodiac) Simplify Listing 3.9 using an array of strings to store
**7.34
the animal names.
(Sort characters in a string) Write a method that returns a sorted string using the
following header:
public static String sort(String s)
***7.35
For example, sort("acb") returns abc.
Write a test program that prompts the user to enter a string and displays the sorted
string.
(Game: hangman) Write a hangman game that randomly generates a word and
prompts the user to guess one letter at a time, as shown in the sample run. Each
letter in the word is displayed as an asterisk. When the user makes a correct
guess, the actual letter is then displayed. When the user finishes a word, display
Programming Exercises 285
the number of misses and ask the user whether to continue to play with another
word. Declare an array to store words, as follows:
// Add any words you wish in this array
String[] words = {"write", "that", ...};
(Guess) Enter a letter in word ******* > p
(Guess) Enter a letter in word p****** > r
(Guess) Enter a letter in word pr**r** > p
p is already in the word
(Guess) Enter a letter in word pr**r** > o
(Guess) Enter a letter in word pro*r** > g
(Guess) Enter a letter in word progr** > n
n is not in the word
(Guess) Enter a letter in word progr** > m
(Guess) Enter a letter in word progr*m > a
The word is program. You missed 1 time
Do you want to guess another word? Enter y or n>
This page intentionally left blank
CHAPTER
8
MULTIDIMENSIONAL
ARRAYS
Objectives
■
To give examples of representing data using two-dimensional arrays
(§8.1).
■
To declare variables for two-dimensional arrays, create arrays, and
access array elements in a two-dimensional array using row and column indexes (§8.2).
■
To program common operations for two-dimensional arrays (displaying arrays, summing all elements, finding the minimum and maximum
elements, and random shuffling) (§8.3).
■
To pass two-dimensional arrays to methods (§8.4).
■
To write a program for grading multiple-choice questions using twodimensional arrays (§8.5).
■
To solve the closest-pair problem using two-dimensional arrays (§8.6).
■
To check a Sudoku solution using two-dimensional arrays (§8.7).
■
To use multidimensional arrays (§8.8).
288 Chapter 8
Multidimensional Arrays
8.1 Introduction
Key
Point
problem
Data in a table or a matrix can be represented using a two-dimensional array.
The preceding chapter introduced how to use one-dimensional arrays to store linear collections of elements. You can use a two-dimensional array to store a matrix or a table. For
example, the following table that lists the distances between cities can be stored using a twodimensional array named distances.
Distance Table (in miles)
Chicago
Boston
New York
Atlanta
Miami
Dallas
Houston
Chicago
Boston
New York
Atlanta
Miami
Dallas
Houston
0
983
787
714
1375
967
1087
983
0
214
1102
1763
1723
1842
787
214
0
888
1549
1548
1627
714
1102
888
0
661
781
810
1375
1763
1549
661
0
1426
1187
967
1723
1548
781
1426
0
239
1087
1842
1627
810
1187
239
0
double[][] distances = {
{0, 983, 787, 714, 1375, 967, 1087},
{983, 0, 214, 1102, 1763, 1723, 1842},
{787, 214, 0, 888, 1549, 1548, 1627},
{714, 1102, 888, 0, 661, 781, 810},
{1375, 1763, 1549, 661, 0, 1426, 1187},
{967, 1723, 1548, 781, 1426, 0, 239},
{1087, 1842, 1627, 810, 1187, 239, 0},
};
8.2 Two-Dimensional Array Basics
Key
Point
An element in a two-dimensional array is accessed through a row and column index.
How do you declare a variable for two-dimensional arrays? How do you create a twodimensional array? How do you access elements in a two-dimensional array? This section
addresses these issues.
8.2.1
Declaring Variables of Two-Dimensional Arrays and Creating
Two-Dimensional Arrays
The syntax for declaring a two-dimensional array is:
elementType[][] arrayRefVar;
or
elementType arrayRefVar[][]; // Allowed, but not preferred
As an example, here is how you would declare a two-dimensional array variable matrix
of int values:
int[][] matrix;
8.2 Two-Dimensional Array Basics 289
or
int matrix[][]; // This style is allowed, but not preferred
You can create a two-dimensional array of 5-by-5 int values and assign it to matrix
using this syntax:
matrix = new int[5][5];
Two subscripts are used in a two-dimensional array, one for the row and the other for the
column. As in a one-dimensional array, the index for each subscript is of the int type and
starts from 0, as shown in Figure 8.1a.
[0][1][2][3][4]
[0][1][2][3][4]
[0][1][2]
[0] 0
0
0
0
0
[0] 0
0
0
0
0
[0] 1
2
3
[1] 0
0
0
0
0
[1] 0
0
0
0
0
[1] 4
5
6
[2] 0
0
0
0
0
[2] 0
7
0
0
0
[2] 7
8
9
[3] 0
0
0
0
0
[3] 0
0
0
0
0
[3] 10 11 12
[4] 0
0
0
0
0
[4] 0
0
0
0
0
matrix = new int[5][5];
matrix[2][1] = 7;
(a)
(b)
int[][] array = {
{1, 2, 3},
{4, 5, 6},
{7, 8, 9},
{10, 11, 12}
};
(c)
FIGURE 8.1 The index of each subscript of a two-dimensional array is an int value,
starting from 0.
To assign the value 7 to a specific element at row 2 and column 1, as shown in Figure 8.1b,
you can use the following syntax:
matrix[2][1] = 7;
Caution
It is a common mistake to use matrix[2, 1] to access the element at row 2 and
column 1. In Java, each subscript must be enclosed in a pair of square brackets.
You can also use an array initializer to declare, create, and initialize a two-dimensional
array. For example, the following code in (a) creates an array with the specified initial values,
as shown in Figure 8.1c. This is equivalent to the code in (b).
int[][] array = {
{1, 2, 3},
{4, 5, 6},
{7, 8, 9},
{10, 11, 12}
};
(a)
8.2.2
Equivalent
int[][] array
array[0][0] =
array[1][0] =
array[2][0] =
array[3][0] =
= new int[4][3];
1; array[0][1] = 2; array[0][2] =
4; array[1][1] = 5; array[1][2] =
7; array[2][1] = 8; array[2][2] =
10; array[3][1] = 11; array[3][2]
(b)
Obtaining the Lengths of Two-Dimensional Arrays
A two-dimensional array is actually an array in which each element is a one-dimensional
array. The length of an array x is the number of elements in the array, which can be obtained
using x.length. x[0], x[1], . . . , and x[x.length-1] are arrays. Their lengths can be
obtained using x[0].length, x[1].length, . . . , and x[x.length-1].length.
3;
6;
9;
= 12;
290 Chapter 8
Multidimensional Arrays
For example, suppose x = new int[3][4], x[0], x[1], and x[2] are one-dimensional
arrays and each contains four elements, as shown in Figure 8.2. x.length is 3, and
x[0].length, x[1].length, and x[2].length are 4.
x
x[0][0] x[0][1] x[0][2] x[0][3]
x[0].length is 4
x[1][0] x[1][1] x[1][2] x[1][3]
x[1].length is 4
x[2][0] x[2][1] x[2][2] x[2][3]
x[2].length is 4
x[0]
x[1]
x[2]
x.length is 3
FIGURE 8.2 A two-dimensional array is a one-dimensional array in which each element is
another one-dimensional array.
8.2.3
ragged array
Ragged Arrays
Each row in a two-dimensional array is itself an array. Thus, the rows can have different
lengths. An array of this kind is known as a ragged array. Here is an example of creating a
ragged array:
int[][] triangleArray = {
{1, 2, 3, 4, 5},
{2, 3, 4, 5},
{3, 4, 5},
{4, 5},
{5}
};
1 2 3 4 5
2 3 4 5
3 4 5
4 5
5
As you can see, triangleArray[0].length is 5, triangleArray[1].length is 4,
triangleArray[2].length is 3, triangleArray[3].length is 2, and triangleArray[4].length is 1.
If you don’t know the values in a ragged array in advance, but do know the sizes—say, the
same as before—you can create a ragged array using the following syntax:
int[][] triangleArray = new int[5][];
triangleArray[0] = new int[5];
triangleArray[1] = new int[4];
triangleArray[2] = new int[3];
triangleArray[3] = new int[2];
triangleArray[4] = new int[1];
You can now assign values to the array. For example,
triangleArray[0][3] = 50;
triangleArray[4][0] = 45;
Note
The syntax new int[5][] for creating an array requires the first index to be specified.
The syntax new int[][] would be wrong.
8.3 Processing Two-Dimensional Arrays 291
8.1
Declare an array reference variable for a two-dimensional array of int values, create
a 4-by-5 int matrix, and assign it to the variable.
8.2
8.3
Can the rows in a two-dimensional array have different lengths?
What is the output of the following code?
✓
Check
Point
int[][] array = new int[5][6];
int[] x = {1, 2};
array[0] = x;
System.out.println("array[0][1] is " + array[0][1]);
8.4
Which of the following statements are valid?
int[][]
int[] x
int[][]
int[][]
int[][]
int[][]
r
=
y
z
m
n
= new int[2];
new int[];
= new int[3][];
= {{1, 2}};
= {{1, 2}, {2, 3}};
= {{1, 2}, {2, 3}, };
8.3 Processing Two-Dimensional Arrays
Nested for loops are often used to process a two-dimensional array.
Suppose an array matrix is created as follows:
int[][] matrix = new int[10][10];
The following are some examples of processing two-dimensional arrays.
1. Initializing arrays with input values. The following loop initializes the array with user
input values:
java.util.Scanner input = new Scanner(System.in);
System.out.println("Enter " + matrix.length + " rows and " +
matrix[0].length + " columns: ");
for (int row = 0; row < matrix.length; row++) {
for (int column = 0; column < matrix[row].length; column++) {
matrix[row][column] = input.nextInt();
}
}
2. Initializing arrays with random values. The following loop initializes the array with
random values between 0 and 99:
for (int row = 0; row < matrix.length; row++) {
for (int column = 0; column < matrix[row].length; column++) {
matrix[row][column] = (int)(Math.random() * 100);
}
}
3. Printing arrays. To print a two-dimensional array, you have to print each element in the
array using a loop like the following:
for (int row = 0; row < matrix.length; row++) {
for (int column = 0; column < matrix[row].length; column++) {
System.out.print(matrix[row][column] + " ");
}
System.out.println();
}
Key
Point
292 Chapter 8
Multidimensional Arrays
4. Summing all elements. Use a variable named total to store the sum. Initially total is
0. Add each element in the array to total using a loop like this:
int total = 0;
for (int row = 0; row < matrix.length; row++) {
for (int column = 0; column < matrix[row].length; column++) {
total += matrix[row][column];
}
}
5. Summing elements by column. For each column, use a variable named total to store
its sum. Add each element in the column to total using a loop like this:
for (int column = 0; column < matrix[0].length; column++) {
int total = 0;
for (int row = 0; row < matrix.length; row++)
total += matrix[row][column];
System.out.println("Sum for column " + column + " is "
+ total);
}
VideoNote
Find the row with the largest
sum
6. Which row has the largest sum? Use variables maxRow and indexOfMaxRow to track
the largest sum and index of the row. For each row, compute its sum and update maxRow
and indexOfMaxRow if the new sum is greater.
int maxRow = 0;
int indexOfMaxRow = 0;
// Get sum of the first row in maxRow
for (int column = 0; column < matrix[0].length; column++) {
maxRow += matrix[0][column];
}
for (int row = 1; row < matrix.length; row++) {
int totalOfThisRow = 0;
for (int column = 0; column < matrix[row].length; column++)
totalOfThisRow += matrix[row][column];
if (totalOfThisRow > maxRow) {
maxRow = totalOfThisRow;
indexOfMaxRow = row;
}
}
System.out.println("Row " + indexOfMaxRow
+ " has the maximum sum of " + maxRow);
7. Random shuffling. Shuffling the elements in a one-dimensional array was introduced
in Section 7.2.6. How do you shuffle all the elements in a two-dimensional array? To
accomplish this, for each element matrix[i][j], randomly generate indices i1 and
j1 and swap matrix[i][j] with matrix[i1][j1], as follows:
for (int i
for (int
int i1
int j1
=
j
=
=
0; i < matrix.length; i++) {
= 0; j < matrix[i].length; j++) {
(int)(Math.random() * matrix.length);
(int)(Math.random() * matrix[i].length);
// Swap matrix[i][j] with matrix[i1][j1]
8.4 Passing Two-Dimensional Arrays to Methods 293
int temp = matrix[i][j];
matrix[i][j] = matrix[i1][j1];
matrix[i1][j1] = temp;
}
}
8.5
Show the output of the following code:
int[][] array = {{1, 2}, {3, 4},
for (int i = array.length - 1; i
for (int j = array[i].length System.out.print(array[i][j]
System.out.println();
}
8.6
{5, 6}};
>= 0; i——) {
1; j >= 0; j——)
+ " ");
✓
Check
Point
Show the output of the following code:
int[][] array = {{1, 2}, {3, 4}, {5, 6}};
int sum = 0;
for (int i = 0; i < array.length; i++)
sum += array[i][0];
System.out.println(sum);
8.4 Passing Two-Dimensional Arrays to Methods
When passing a two-dimensional array to a method, the reference of the array is
passed to the method.
Key
Point
You can pass a two-dimensional array to a method just as you pass a one-dimensional array.
You can also return an array from a method. Listing 8.1 gives an example with two methods.
The first method, getArray(), returns a two-dimensional array, and the second method,
sum(int[][] m), returns the sum of all the elements in a matrix.
LISTING 8.1 PassTwoDimensionalArray.java
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
import java.util.Scanner;
public class PassTwoDimensionalArray {
public static void main(String[] args) {
int[][] m = getArray(); // Get an array
// Display sum of elements
System.out.println("\nSum of all elements is " + sum(m));
get array
pass array
}
public static int[][] getArray() {
// Create a Scanner
Scanner input = new Scanner(System.in);
// Enter array values
int[][] m = new int[3][4];
System.out.println("Enter " + m.length + " rows and "
+ m[0].length + " columns: ");
for (int i = 0; i < m.length; i++)
for (int j = 0; j < m[i].length; j++)
m[i][j] = input.nextInt();
getArray method
294 Chapter 8
Multidimensional Arrays
23
24
25
26
27
28
29
30
31
32
33
34
35
36
return array
sum method
return m;
}
public static int sum(int[][] m) {
int total = 0;
for (int row = 0; row < m.length; row++) {
for (int column = 0; column < m[row].length; column++) {
total += m[row][column];
}
}
return total;
}
}
Enter 3 rows and 4 columns:
1 2 3 4
5 6 7 8
9 10 11 12
Sum of all elements is 78
The method getArray prompts the user to enter values for the array (lines 11–24) and
returns the array (line 23).
The method sum (lines 26–35) has a two-dimensional array argument. You can obtain the
number of rows using m.length (line 28) and the number of columns in a specified row using
m[row].length (line 29).
✓
Check
Point
8.7
Show the output of the following code:
public class Test {
public static void main(String[] args) {
int[][] array = {{1, 2, 3, 4}, {5, 6, 7, 8}};
System.out.println(m1(array)[0]);
System.out.println(m1(array)[1]);
}
public static int[] m1(int[][] m) {
int[] result = new int[2];
result[0] = m.length;
result[1] = m[0].length;
return result;
}
}
8.5 Case Study: Grading a Multiple-Choice Test
Key
Point
VideoNote
Grade multiple-choice test
The problem is to write a program that will grade multiple-choice tests.
Suppose you need to write a program that grades multiple-choice tests. Assume there are eight
students and ten questions, and the answers are stored in a two-dimensional array. Each row
records a student’s answers to the questions, as shown in the following array.
8.5 Case Study: Grading a Multiple-Choice Test 295
Students’ Answers to the Questions:
0 1 2 3 4 5 6 7 8 9
Student
Student
Student
Student
Student
Student
Student
Student
0
1
2
3
4
5
6
7
A
D
E
C
A
B
B
E
B
B
D
B
B
B
B
B
A
A
D
A
D
E
A
E
C
B
A
E
C
C
C
C
C
C
C
D
C
C
C
C
D
A
B
C
D
D
D
D
E
E
E
E
E
E
E
E
E
E
E
E
E
E
E
E
A
A
A
A
A
A
A
A
D
D
D
D
D
D
D
D
The key is stored in a one-dimensional array:
Key to the Questions:
0 1 2 3 4 5 6 7 8 9
Key D B D C C D A E A D
Your program grades the test and displays the result. It compares each student’s answers
with the key, counts the number of correct answers, and displays it. Listing 8.2 gives the
program.
LISTING 8.2 GradeExam.java
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
public class GradeExam {
/** Main method */
public static void main(String[] args) {
// Students' answers to the questions
char[][] answers = {
{'A', 'B', 'A', 'C', 'C', 'D', 'E', 'E',
{'D', 'B', 'A', 'B', 'C', 'A', 'E', 'E',
{'E', 'D', 'D', 'A', 'C', 'B', 'E', 'E',
{'C', 'B', 'A', 'E', 'D', 'C', 'E', 'E',
{'A', 'B', 'D', 'C', 'C', 'D', 'E', 'E',
{'B', 'B', 'E', 'C', 'C', 'D', 'E', 'E',
{'B', 'B', 'A', 'C', 'C', 'D', 'E', 'E',
{'E', 'B', 'E', 'C', 'C', 'D', 'E', 'E',
2-D array
'A',
'A',
'A',
'A',
'A',
'A',
'A',
'A',
'D'},
'D'},
'D'},
'D'},
'D'},
'D'},
'D'},
'D'}};
// Key to the questions
char[] keys = {'D', 'B', 'D', 'C', 'C', 'D', 'A', 'E', 'A', 'D'};
// Grade all answers
for (int i = 0; i < answers.length; i++) {
// Grade one student
int correctCount = 0;
for (int j = 0; j < answers[i].length; j++) {
if (answers[i][j] == keys[j])
correctCount++;
}
System.out.println("Student " + i + "'s correct count is " +
correctCount);
}
}
}
1-D array
compare with key
296 Chapter 8
Multidimensional Arrays
Student
Student
Student
Student
Student
Student
Student
Student
0's
1's
2's
3's
4's
5's
6's
7's
correct
correct
correct
correct
correct
correct
correct
correct
count
count
count
count
count
count
count
count
is
is
is
is
is
is
is
is
7
6
5
4
8
7
7
7
The statement in lines 5–13 declares, creates, and initializes a two-dimensional array of
characters and assigns the reference to answers of the char[][] type.
The statement in line 16 declares, creates, and initializes an array of char values and
assigns the reference to keys of the char[] type.
Each row in the array answers stores a student’s answer, which is graded by comparing
it with the key in the array keys. The result is displayed immediately after a student’s answer
is graded.
8.6 Case Study: Finding the Closest Pair
Key
Point
closest-pair animation on the
Companion Website
This section presents a geometric problem for finding the closest pair of points.
Given a set of points, the closest-pair problem is to find the two points that are nearest to
each other. In Figure 8.3, for example, points (1, 1) and (2, 0.5) are closest to each
other. There are several ways to solve this problem. An intuitive approach is to compute the
distances between all pairs of points and find the one with the minimum distance, as implemented in Listing 8.3.
(–1, 3)
(3, 3)
(4, 2)
(1, 1)
(2, 0.5)
(4, –0.5)
(–1, –1)
FIGURE 8.3
(2, –1)
x
0 –1
1 –1
2 1
3 2
4 2
5 3
6 4
7 4
Points can be represented in a two-dimensional array.
LISTING 8.3 FindNearestPoints.java
number of points
1
2
3
4
5
6
7
8
9
import java.util.Scanner;
public class FindNearestPoints {
public static void main(String[] args) {
Scanner input = new Scanner(System.in);
System.out.print("Enter the number of points: ");
int numberOfPoints = input.nextInt();
// Create an array to store points
y
3
–1
1
0.5
–1
3
2
–0.5
8.6 Case Study: Finding the Closest Pair 297
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
double[][] points = new double[numberOfPoints][2];
System.out.print("Enter " + numberOfPoints + " points: ");
for (int i = 0; i < points.length; i++) {
points[i][0] = input.nextDouble();
points[i][1] = input.nextDouble();
}
// p1 and p2 are the indices in the points' array
int p1 = 0, p2 = 1; // Initial two points
double shortestDistance = distance(points[p1][0], points[p1][1],
points[p2][0], points[p2][1]); // Initialize shortestDistance
// Compute distance for every two points
for (int i = 0; i < points.length; i++) {
for (int j = i + 1; j < points.length; j++) {
double distance = distance(points[i][0], points[i][1],
points[j][0], points[j][1]); // Find distance
if (shortestDistance > distance) {
p1 = i; // Update p1
p2 = j; // Update p2
shortestDistance = distance; // Update shortestDistance
}
}
}
// Display result
System.out.println("The closest two points are " +
"(" + points[p1][0] + ", " + points[p1][1] + ") and (" +
points[p2][0] + ", " + points[p2][1] + ")");
}
/** Compute the distance between two points (x1, y1) and (x2, y2)*/
public static double distance(
double x1, double y1, double x2, double y2) {
return Math.sqrt((x2 - x1) * (x2 - x1) + (y2 - y1) * (y2 - y1));
}
}
Enter the number of points: 8
Enter 8 points: -1 3 -1 -1 1 1 2 0.5 2 -1 3 3
The closest two points are (1, 1) and (2, 0.5)
4 2 4 -0.5
The program prompts the user to enter the number of points (lines 6–7). The points are
read from the console and stored in a two-dimensional array named points (lines 12–15).
The program uses the variable shortestDistance (line 19) to store the distance between
the two nearest points, and the indices of these two points in the points array are stored in
p1 and p2 (line 18).
For each point at index i, the program computes the distance between points[i] and
points[j] for all j > i (lines 23–34). Whenever a shorter distance is found, the variable
shortestDistance and p1 and p2 are updated (lines 28–32).
The distance between two points (x1, y1) and (x2, y2) can be computed using the
formula 2(x2 - x1)2 + (y2 - y1)2 (lines 43–46).
The program assumes that the plane has at least two points. You can easily modify the
program to handle the case if the plane has zero or one point.
2-D array
read points
track two points
track shortestDistance
for each point i
for each point j
distance between i and j
distance between two points
update shortestDistance
298 Chapter 8
Multidimensional Arrays
Note that there might be more than one closest pair of points with the same minimum distance. The program finds one such pair. You may modify the program to find all closest pairs
in Programming Exercise 8.8.
multiple closest pairs
Tip
It is cumbersome to enter all points from the keyboard. You may store the input in a file, say
FindNearestPoints.txt, and compile and run the program using the following command:
input file
java FindNearestPoints < FindNearestPoints.txt
8.7 Case Study: Sudoku
Key
Point
VideoNote
Sudoku
fixed cells
free cells
The problem is to check whether a given Sudoku solution is correct.
This section presents an interesting problem of a sort that appears in the newspaper every
day. It is a number-placement puzzle, commonly known as Sudoku. This is a very challenging problem. To make it accessible to the novice, this section presents a simplified version of
the Sudoku problem, which is to verify whether a Sudoku solution is correct. The complete
program for finding a Sudoku solution is presented in Supplement VI.A.
Sudoku is a 9 * 9 grid divided into smaller 3 * 3 boxes (also called regions or blocks), as
shown in Figure 8.4a. Some cells, called fixed cells, are populated with numbers from 1 to 9. The
objective is to fill the empty cells, also called free cells, with the numbers 1 to 9 so that every
row, every column, and every 3 * 3 box contains the numbers 1 to 9, as shown in Figure 8.4b.
5
3
7
6
1
9
9
8
3
4
6
7
8
9
1
2
6
7
2
1
9
5
3
4
8
1
9
8
3
4
2
5
6
7
8
5
9
7
6
1
4
2
3
4
2
6
8
5
3
7
9
1
7
1
3
9
2
4
8
5
6
9
6
1
5
3
7
2
8
4
5
2
8
7
4
1
9
6
3
5
9
3
4
5
2
8
6
1
7
9
6
8
6
4
5
5
8
7
3
3
Solution
1
2
6
6
4
1
9
8
7
(a) Puzzle
FIGURE 8.4
representing a grid
(b) Solution
The Sudoku puzzle in (a) is solved in (b).
For convenience, we use value 0 to indicate a free cell, as shown in Figure 8.5a. The grid
can be naturally represented using a two-dimensional array, as shown in Figure 8.5b.
5
3
0
0
7
0
0
0
0
6
0
0
1
9
5
0
0
0
0
9
8
0
0
0
0
6
0
8
0
0
0
6
0
0
0
3
4
0
0
8
0
3
0
0
1
7
0
0
0
2
0
0
0
6
0
6
0
0
0
0
0
0
0
0
0
0
4
1
9
0
0
5
0
0
0
0
8
0
0
7
9
(a)
FIGURE 8.5
int[][] grid =
{{5, 3, 0, 0,
{6, 0, 0, 1,
{0, 9, 8, 0,
{8, 0, 0, 0,
{4, 0, 0, 8,
{7, 0, 0, 0,
{0, 6, 0, 0,
{0, 0, 0, 4,
{0, 0, 0, 0,
};
7,
9,
0,
6,
0,
2,
0,
1,
8,
0,
5,
0,
0,
3,
0,
0,
9,
0,
0,
0,
0,
0,
0,
0,
2,
0,
0,
(b)
A grid can be represented using a two-dimensional array.
0,
0,
6,
0,
0,
0,
8,
0,
7,
0},
0},
0},
3},
1},
6},
0},
5},
9}
8.7 Case Study: Sudoku 299
To find a solution for the puzzle, we must replace each 0 in the grid with an appropriate
number from 1 to 9. For the solution to the puzzle in Figure 8.5, the grid should be as shown
in Figure 8.6.
Once a solution to a Sudoku puzzle is found, how do you verify that it is correct? Here are
two approaches:
■
Check if every row has numbers from 1 to 9, every column has numbers from 1 to 9,
and every small box has numbers from 1 to 9.
■
Check each cell. Each cell must be a number from 1 to 9 and the cell must be unique
on every row, every column, and every small box.
A solution grid is
{{5, 3, 4, 6, 7, 8,
{6, 7, 2, 1, 9, 5,
{1, 9, 8, 3, 4, 2,
{8, 5, 9, 7, 6, 1,
{4, 2, 6, 8, 5, 3,
{7, 1, 3, 9, 2, 4,
{9, 6, 1, 5, 3, 7,
{2, 8, 7, 4, 1, 9,
{3, 4, 5, 2, 8, 6,
};
9,
3,
5,
4,
7,
8,
2,
6,
1,
1,
4,
6,
2,
9,
5,
8,
3,
7,
2},
8},
7},
3},
1},
6},
4},
5},
9}
FIGURE 8.6 A solution is stored in grid.
The program in Listing 8.4 prompts the user to enter a solution and reports whether it is
valid. We use the second approach in the program to check whether the solution is correct.
LISTING 8.4 CheckSudokuSolution.java
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
import java.util.Scanner;
public class CheckSudokuSolution {
public static void main(String[] args) {
// Read a Sudoku solution
int[][] grid = readASolution();
System.out.println(isValid(grid) ? "Valid solution" :
"Invalid solution");
read input
solution valid?
}
/** Read a Sudoku solution from the console */
public static int[][] readASolution() {
// Create a Scanner
Scanner input = new Scanner(System.in);
read solution
System.out.println("Enter a Sudoku puzzle solution:");
int[][] grid = new int[9][9];
for (int i = 0; i < 9; i++)
for (int j = 0; j < 9; j++)
grid[i][j] = input.nextInt();
return grid;
}
/** Check whether a solution is valid */
public static boolean isValid(int[][] grid) {
check solution
300 Chapter 8
Multidimensional Arrays
check rows
check columns
check small boxes
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
for (int i = 0; i < 9; i++)
for (int j = 0; j < 9; j++)
if (grid[i][j] < 1 || grid[i][j] > 9
|| !isValid(i, j, grid))
return false;
return true; // The solution is valid
}
/** Check whether grid[i][j] is valid in the grid */
public static boolean isValid(int i, int j, int[][] grid) {
// Check whether grid[i][j] is unique in i's row
for (int column = 0; column < 9; column++)
if (column != j && grid[i][column] == grid[i][j])
return false;
// Check whether grid[i][j] is unique in j's column
for (int row = 0; row < 9; row++)
if (row != i && grid[row][j] == grid[i][j])
return false;
// Check whether grid[i][j] is unique in the 3-by-3 box
for (int row = (i / 3) * 3; row < (i / 3) * 3 + 3; row++)
for (int col = (j / 3) * 3; col < (j / 3) * 3 + 3; col++)
if (row != i && col != j && grid[row][col] == grid[i][j])
return false;
return true; // The current value at grid[i][j] is valid
}
}
Enter a Sudoku puzzle solution:
9 6 3 1 7 4 2 5 8
1 7 8 3 2 5 6 4 9
2 5 4 6 8 9 7 3 1
8 2 1 4 3 7 5 9 6
4 9 6 8 5 2 3 1 7
7 3 5 9 6 1 8 2 4
5 8 9 7 1 3 4 6 2
3 1 7 2 4 6 9 8 5
6 4 2 5 9 8 1 7 3
Valid solution
isValid method
overloaded isValid method
The program invokes the readASolution() method (line 6) to read a Sudoku solution
and return a two-dimensional array representing a Sudoku grid.
The isValid(grid) method checks whether the values in the grid are valid by verifying
that each value is between 1 and 9 and that each value is valid in the grid (lines 27–34).
The isValid(i, j, grid) method checks whether the value at grid[i][j] is valid.
It checks whether grid[i][j] appears more than once in row i (lines 39–41), in column j
(lines 44–46), and in the 3 * 3 box (lines 49–52).
How do you locate all the cells in the same box? For any grid[i][j], the starting cell
of the 3 * 3 box that contains it is grid[(i / 3) * 3][(j / 3) * 3], as illustrated in
Figure 8.7.
8.8 Multidimensional Arrays 301
grid[0][6]
grid[0][0]
For any grid[i][j] in this 3 by 3 box, its starting cell
is grid[3*(i/3)][3*(j/3)] (i.e., grid[0][6]). For
example, for grid[2][8], i=2 and j=8, 3*(i/3)=0 and
3*(j/3)=6.
grid[6][3]
For any grid[i][j] in this 3 by 3 box, its
starting cell is grid[3*(i/3)][3*(j/3)]
(i.e., grid[6][3]). For example, for
grid[8][5], i=8 and j=5, 3*(i/3)=6 and
3*(j/3)=3.
FIGURE 8.7 The location of the first cell in a 3 * 3 box determines the locations of other cells in the box.
With this observation, you can easily identify all the cells in the box. For instance, if
grid[r][c] is the starting cell of a 3 * 3 box, the cells in the box can be traversed in a
nested loop as follows:
// Get all cells in a 3-by-3 box starting at grid[r][c]
for (int row = r; row < r + 3; row++)
for (int col = c; col < c + 3; col++)
// grid[row][col] is in the box
It is cumbersome to enter 81 numbers from the console. When you test the program, you
may store the input in a file, say CheckSudokuSolution.txt (see www.cs.armstrong.edu/liang/
data/CheckSudokuSolution.txt), and run the program using the following command:
input file
java CheckSudokuSolution < CheckSudokuSolution.txt
8.8 Multidimensional Arrays
A two-dimensional array consists of an array of one-dimensional arrays and a threedimensional array consists of an array of two-dimensional arrays.
In the preceding section, you used a two-dimensional array to represent a matrix or a table.
Occasionally, you will need to represent n-dimensional data structures. In Java, you can create
n-dimensional arrays for any integer n.
The way to declare two-dimensional array variables and create two-dimensional arrays can
be generalized to declare n-dimensional array variables and create n-dimensional arrays for
n 7 = 3. For example, you may use a three-dimensional array to store exam scores for a class
of six students with five exams, and each exam has two parts (multiple-choice and essay). The
following syntax declares a three-dimensional array variable scores, creates an array, and
assigns its reference to scores.
double[][][] scores = new double[6][5][2];
You can also use the short-hand notation to create and initialize the array as follows:
double[][][] scores =
{{7.5, 20.5}, {9.0,
{{4.5, 21.5}, {9.0,
{{6.5, 30.5}, {9.4,
{{6.5, 23.5}, {9.4,
{{8.5, 26.5}, {9.4,
{{9.5, 20.5}, {9.4,
{
22.5},
22.5},
10.5},
32.5},
52.5},
42.5},
{15,
{15,
{11,
{13,
{13,
{13,
33.5},
34.5},
33.5},
34.5},
36.5},
31.5},
{13,
{12,
{11,
{11,
{13,
{12,
21.5},
20.5},
23.5},
20.5},
24.5},
20.5},
{15,
{14,
{10,
{16,
{16,
{16,
2.5}},
9.5}},
2.5}},
7.5}},
2.5}},
6.5}}};
Key
Point
302 Chapter 8
Multidimensional Arrays
scores[0][1][0] refers to the multiple-choice score for the first student’s second exam,
which is 9.0. scores[0][1][1] refers to the essay score for the first student’s second
exam, which is 22.5. This is depicted in the following figure:
Which student
Which exam
scores [i]
[j]
Multiple-choice or essay
[k]
A multidimensional array is actually an array in which each element is another array. A threedimensional array consists of an array of two-dimensional arrays. A two-dimensional array
consists of an array of one-dimensional arrays. For example, suppose x = new int[2]
[2][5], and x[0] and x[1] are two-dimensional arrays. X[0][0], x[0][1], x[1][0],
and x[1][1] are one-dimensional arrays and each contains five elements. x.length
is 2, x[0].length and x[1].length are 2, and X[0][0].length, x[0][1].length,
x[1][0].length, and x[1][1].length are 5.
8.8.1
Case Study: Daily Temperature and Humidity
Suppose a meteorology station records the temperature and humidity every hour of every
day and stores the data for the past ten days in a text file named Weather.txt (see www
.cs.armstrong.edu/liang/data/Weather.txt). Each line of the file consists of four numbers that
indicate the day, hour, temperature, and humidity. The contents of the file may look like
the one in (a).
Day
Temperature
Hour
1
1
. . .
10
10
Day
Humidity
1
2
76.4
77.7
0.92
0.93
23
24
97.7
98.7
0.71
0.74
Temperature
Hour
10
1
. . .
10
1
(a)
Humidity
24
2
98.7
77.7
0.74
0.93
23
1
97.7
76.4
0.71
0.92
(b)
Note that the lines in the file are not necessarily in increasing order of day and hour. For example, the file may appear as shown in (b).
Your task is to write a program that calculates the average daily temperature and humidity for the 10 days. You can use the input redirection to read the file and store the data in
a three-dimensional array named data. The first index of data ranges from 0 to 9 and
represents 10 days, the second index ranges from 0 to 23 and represents 24 hours, and the
third index ranges from 0 to 1 and represents temperature and humidity, as depicted in the
following figure:
Which day
Which hour
data [ i ] [ j ] [ k ]
Temperature or humidity
8.8 Multidimensional Arrays 303
Note that the days are numbered from 1 to 10 and the hours from 1 to 24 in the file.
Because the array index starts from 0, data[0][0][0] stores the temperature in day 1 at
hour 1 and data[9][23][1] stores the humidity in day 10 at hour 24.
The program is given in Listing 8.5.
LISTING 8.5 Weather.java
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
import java.util.Scanner;
public class Weather {
public static void main(String[] args) {
final int NUMBER_OF_DAYS = 10;
final int NUMBER_OF_HOURS = 24;
double[][][] data
= new double[NUMBER_OF_DAYS][NUMBER_OF_HOURS][2];
Scanner input = new Scanner(System.in);
// Read input using input redirection from a file
for (int k = 0; k < NUMBER_OF_DAYS * NUMBER_OF_HOURS; k++) {
int day = input.nextInt();
int hour = input.nextInt();
double temperature = input.nextDouble();
double humidity = input.nextDouble();
data[day - 1][hour - 1][0] = temperature;
data[day - 1][hour - 1][1] = humidity;
}
// Find the average daily temperature and humidity
for (int i = 0; i < NUMBER_OF_DAYS; i++) {
double dailyTemperatureTotal = 0, dailyHumidityTotal = 0;
for (int j = 0; j < NUMBER_OF_HOURS; j++) {
dailyTemperatureTotal += data[i][j][0];
dailyHumidityTotal += data[i][j][1];
}
// Display result
System.out.println("Day " + i + "'s average temperature is "
+ dailyTemperatureTotal / NUMBER_OF_HOURS);
System.out.println("Day " + i + "'s average humidity is "
+ dailyHumidityTotal / NUMBER_OF_HOURS);
}
}
}
Day
Day
Day
Day
. .
Day
Day
0's
0's
1's
1's
.
9's
9's
average
average
average
average
temperature
humidity is
temperature
humidity is
is 77.7708
0.929583
is 77.3125
0.929583
average temperature is 79.3542
average humidity is 0.9125
You can use the following command to run the program:
java Weather < Weather.txt
A three-dimensional array for storing temperature and humidity is created in line 8. The
loop in lines 12–19 reads the input to the array. You can enter the input from the keyboard, but
three-dimensional array
304 Chapter 8
Multidimensional Arrays
doing so will be awkward. For convenience, we store the data in a file and use input redirection to read the data from the file. The loop in lines 24–27 adds all temperatures for each hour
in a day to dailyTemperatureTotal and all humidity for each hour to dailyHumidityTotal. The average daily temperature and humidity are displayed in lines 30–33.
8.8.2
Case Study: Guessing Birthdays
Listing 3.3, GuessBirthday.java, gives a program that guesses a birthday. The program can be
simplified by storing the numbers in five sets in a three-dimensional array, and it prompts the
user for the answers using a loop, as shown in Listing 8.6. The sample run of the program can
be the same as shown in Listing 4.3.
LISTING 8.6 GuessBirthdayUsingArray.java
three-dimensional array
Set i
add to day
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
import java.util.Scanner;
public class GuessBirthdayUsingArray {
public static void main(String[] args) {
int day = 0; // Day to be determined
int answer;
int[][][] dates
{{ 1, 3, 5,
{ 9, 11, 13,
{17, 19, 21,
{25, 27, 29,
{{ 2, 3, 6,
{10, 11, 14,
{18, 19, 22,
{26, 27, 30,
{{ 4, 5, 6,
{12, 13, 14,
{20, 21, 22,
{28, 29, 30,
{{ 8, 9, 10,
{12, 13, 14,
{24, 25, 26,
{28, 29, 30,
{{16, 17, 18,
{20, 21, 22,
{24, 25, 26,
{28, 29, 30,
= {
7},
15},
23},
31}},
7},
15},
23},
31}},
7},
15},
23},
31}},
11},
15},
27},
31}},
19},
23},
27},
31}}};
// Create a Scanner
Scanner input = new Scanner(System.in);
for (int i = 0; i < 5; i++) {
System.out.println("Is your birthday in Set" + (i + 1) + "?");
for (int j = 0; j < 4; j++) {
for (int k = 0; k < 4; k++)
System.out.printf("%4d", dates[i][j][k]);
System.out.println();
}
System.out.print("\nEnter 0 for No and 1 for Yes: ");
answer = input.nextInt();
if (answer == 1)
day += dates[i][0][0];
Programming Exercises 305
46
47
48
49
50
}
System.out.println("Your birthday is " + day);
}
}
A three-dimensional array dates is created in Lines 8–28. This array stores five sets of
numbers. Each set is a 4-by-4 two-dimensional array.
The loop starting from line 33 displays the numbers in each set and prompts the user to
answer whether the birthday is in the set (lines 41–42). If the day is in the set, the first number
(dates[i][0][0]) in the set is added to variable day (line 45).
8.8
Declare an array variable for a three-dimensional array, create a 4 * 6 * 5 int
array, and assign its reference to the variable.
8.9
Assume int[][][] x = new char[12][5][2], how many elements are in the
array? What are x.length, x[2].length, and x[0][0].length?
Show the output of the following code:
8.10
int[][][] array = {{{1, 2}, {3, 4}}, {{5, 6},{7, 8}}};
System.out.println(array[0][0][0]);
System.out.println(array[1][1][1]);
CHAPTER SUMMARY
1. A two-dimensional array can be used to store a table.
2. A variable for two-dimensional arrays can be declared using the syntax:
elementType[][] arrayVar.
3. A two-dimensional array can be created using the syntax:
new
elementType
[ROW_SIZE][COLUMN_SIZE].
4. Each element in a two-dimensional array is represented using the syntax:
arrayVar[rowIndex][columnIndex].
5. You can create and initialize a two-dimensional array using an array initializer with the
syntax: elementType[][] arrayVar = {{row values}, . . . , {row values}}.
6. You can use arrays of arrays to form multidimensional arrays. For example, a variable
for three-dimensional arrays can be declared as elementType[][][] arrayVar, and
a three-dimensional array can be created using new elementType[size1][size2]
[size3].
QUIZ
Answer the quiz for this chapter online at www.cs.armstrong.edu/liang/intro10e/quiz.html.
PROGRAMMING EXERCISES
*8.1
(Sum elements column by column) Write a method that returns the sum of all the
elements in a specified column in a matrix using the following header:
public static double sumColumn(double[][] m, int columnIndex)
✓
Check
Point
306 Chapter 8
Multidimensional Arrays
Write a test program that reads a 3-by-4 matrix and displays the sum of each
column. Here is a sample run:
Enter a 3-by-4 matrix row by row:
1.5 2 3 4
5.5 6 7 8
9.5
Sum
Sum
Sum
Sum
*8.2
1 3 1
of the
of the
of the
of the
elements
elements
elements
elements
at
at
at
at
column
column
column
column
0
1
2
3
is
is
is
is
16.5
9.0
13.0
13.0
(Sum the major diagonal in a matrix) Write a method that sums all the numbers
in the major diagonal in an n * n matrix of double values using the following
header:
public static double sumMajorDiagonal(double[][] m)
Write a test program that reads a 4-by-4 matrix and displays the sum of all its
elements on the major diagonal. Here is a sample run:
Enter a 4-by-4 matrix row by row:
1 2 3 4.0
5 6.5 7 8
9 10 11 12
13 14 15 16
Sum of the elements in the major diagonal is 34.5
*8.3
**8.4
8.5
(Sort students on grades) Rewrite Listing 8.2, GradeExam.java, to display the
students in increasing order of the number of correct answers.
(Compute the weekly hours for each employee) Suppose the weekly hours for all
employees are stored in a two-dimensional array. Each row records an employee’s seven-day work hours with seven columns. For example, the following
array stores the work hours for eight employees. Write a program that displays
employees and their total hours in decreasing order of the total hours.
Su
M
T
W
Th
F
Sa
Employee 0
2
4
3
4
5
8
8
Employee 1
7
3
4
3
3
4
4
Employee 2
3
3
4
3
3
2
2
Employee 3
9
3
4
7
3
4
1
Employee 4
3
5
4
3
6
3
8
Employee 5
3
4
4
6
3
4
4
Employee 6
3
7
4
8
3
8
4
Employee 7
6
3
5
9
2
7
9
(Algebra: add two matrices) Write a method to add two matrices. The header of
the method is as follows:
public static double[][] addMatrix(double[][] a, double[][] b)
Programming Exercises 307
In order to be added, the two matrices must have the same dimensions and the
same or compatible types of elements. Let c be the resulting matrix. Each element cij is aij + bij. For example, for two 3 * 3 matrices a and b, c is
a11
£ a21
a31
a12
a22
a32
b11
a13
a23 ≥ + £ b21
a33
b31
b12
b22
b32
a11 + b11
b13
b23 ≥ = £ a21 + b21
b33
a31 + b31
a12 + b12
a22 + b22
a32 + b32
a13 + b13
a23 + b23 ≥
a33 + b33
Write a test program that prompts the user to enter two 3 * 3 matrices and
displays their sum. Here is a sample run:
Enter matrix1: 1 2 3 4 5 6 7 8 9
Enter matrix2: 0 2 4 1 4.5 2.2 1.1
The matrices are added as follows
1.0 2.0 3.0
0.0 2.0 4.0
4.0 5.0 6.0 +
1.0 4.5 2.2 =
7.0 8.0 9.0
1.1 4.3 5.2
**8.6
4.3 5.2
1.0 4.0 7.0
5.0 9.5 8.2
8.1 12.3 14.2
(Algebra: multiply two matrices) Write a method to multiply two matrices. The
header of the method is:
public static double[][]
multiplyMatrix(double[][] a, double[][] b)
To multiply matrix a by matrix b, the number of columns in a must be the same as
the number of rows in b, and the two matrices must have elements of the same or
compatible types. Let c be the result of the multiplication. Assume the column size
of matrix a is n. Each element cij is ai1 * b1j + ai2 * b2j + c + ain * bnj.
For example, for two 3 * 3 matrices a and b, c is
a11
£ a21
a31
a12
a22
a32
b11
a13
a23 ≥ * £ b21
a33
b31
b12
b22
b32
c11
b13
b23 ≥ = £ c21
b33
c31
c12
c22
c32
c13
c23 ≥
c33
where cij = ai1 * b1j + ai2 * b2j + ai3 * b3j.
Write a test program that prompts the user to enter two 3 * 3 matrices and displays their product. Here is a sample run:
Enter matrix1: 1 2 3 4 5 6 7 8 9
Enter matrix2: 0 2 4 1 4.5 2.2 1.1 4.3 5.2
The multiplication of the matrices is
1 2 3
0 2.0 4.0
5.3 23.9 24
4 5 6
* 1 4.5 2.2 =
11.6 56.3 58.2
7 8 9
1.1 4.3 5.2
17.9 88.7 92.4
*8.7
(Points nearest to each other) Listing 8.3 gives a program that finds two points in a
two-dimensional space nearest to each other. Revise the program so that it finds two
points in a three-dimensional space nearest to each other. Use a two-dimensional
array to represent the points. Test the program using the following points:
double[][] points = {{-1, 0, 3}, {-1, -1, -1}, {4, 1, 1},
{2, 0.5, 9}, {3.5, 2, -1}, {3, 1.5, 3}, {-1.5, 4, 2},
{5.5, 4, -0.5}};
VideoNote
Multiply two matrices
308 Chapter 8
Multidimensional Arrays
The formula for computing the distance between two points (x1, y1, z1) and
(x2, y2, z2) is 2(x2 - x1)2 + (y2 - y1)2 + (z2 - z1)2.
**8.8
(All closest pairs of points) Revise Listing 8.3, FindNearestPoints.java, to
display all closest pairs of points with the same minimum distance. Here is
a sample run:
Enter the number of points: 8
Enter 8 points: 0 0 1 1 -1 -1 2 2 -2 -2 -3 -3 -4 -4 5 5
The closest two points are (0.0, 0.0) and (1.0, 1.0)
The closest two points are (0.0, 0.0) and (-1.0, -1.0)
The closest two points are (1.0, 1.0) and (2.0, 2.0)
The closest two points are (-1.0, -1.0) and (-2.0, -2.0)
The closest two points are (-2.0, -2.0) and (-3.0, -3.0)
The closest two points are (-3.0, -3.0) and (-4.0, -4.0)
Their distance is 1.4142135623730951
***8.9
(Game: play a tic-tac-toe game) In a game of tic-tac-toe, two players take turns
marking an available cell in a 3 * 3 grid with their respective tokens (either
X or O). When one player has placed three tokens in a horizontal, vertical, or
diagonal row on the grid, the game is over and that player has won. A draw (no
winner) occurs when all the cells on the grid have been filled with tokens and
neither player has achieved a win. Create a program for playing tic-tac-toe.
The program prompts two players to enter an X token and O token alternately. Whenever a token is entered, the program redisplays the board on the
console and determines the status of the game (win, draw, or continue). Here
is a sample run:
——————-——————
|
|
|
|
——————-——————
|
|
|
|
——————-——————
|
|
|
|
——————-——————
Enter a row (0, 1, or 2) for player X: 1
Enter a column (0, 1, or 2) for player X: 1
——————-——————
|
|
|
|
——————-——————
|
| X |
|
——————-——————
|
|
|
|
——————-——————
Enter a row (0, 1, or 2) for player O: 1
Enter a column (0, 1, or 2) for player O: 2
——————-——————
|
|
|
|
——————-——————
|
| X | O |
——————-——————
|
|
|
|
——————-——————
Programming Exercises 309
Enter a row (0, 1, or 2) for player X:
. . .
——————-——————
| X |
|
|
——————-——————
| O | X | O |
——————-——————
|
|
| X |
——————-——————
X player won
*8.10
(Largest row and column) Write a program that randomly fills in 0s and 1s into
a 4-by-4 matrix, prints the matrix, and finds the first row and column with the
most 1s. Here is a sample run of the program:
0011
0011
1101
1010
The largest row index: 2
The largest column index: 2
**8.11
(Game: nine heads and tails) Nine coins are placed in a 3-by-3 matrix with some
face up and some face down. You can represent the state of the coins using a
3-by-3 matrix with values 0 (heads) and 1 (tails). Here are some examples:
0 0 0
0 1 0
0 0 0
1 0 1
0 0 1
1 0 0
1 1 0
1 0 0
0 0 1
1 0 1
1 1 0
1 0 0
1 0 0
1 1 1
1 1 0
Each state can also be represented using a binary number. For example, the preceding matrices correspond to the numbers
000010000 101001100 110100001 101110100 100111110
There are a total of 512 possibilities, so you can use decimal numbers 0, 1, 2, 3,
. . . , and 511 to represent all states of the matrix. Write a program that prompts
the user to enter a number between 0 and 511 and displays the corresponding
matrix with the characters H and T. Here is a sample run:
Enter a number between 0 and 511: 7
H H H
H H H
T T T
The user entered 7, which corresponds to 000000111. Since 0 stands for H and
1 for T, the output is correct.
**8.12
(Financial application: compute tax) Rewrite Listing 3.5, ComputeTax.java,
using arrays. For each filing status, there are six tax rates. Each rate is applied
to a certain amount of taxable income. For example, from the taxable income of
$400,000 for a single filer, $8,350 is taxed at 10%, (33,950 - 8,350) at 15%,
310 Chapter 8
Multidimensional Arrays
(82,250 - 33,950) at 25%, (171,550 - 82,550) at 28%, (372,550 - 82,250) at
33%, and (400,000 - 372,950) at 36%. The six rates are the same for all filing
statuses, which can be represented in the following array:
double[] rates = {0.10, 0.15, 0.25, 0.28, 0.33, 0.35};
The brackets for each rate for all the filing statuses can be represented in a twodimensional array as follows:
int[][] brackets = {
{8350, 33950, 82250, 171550, 372950}, // Single filer
{16700, 67900, 137050, 20885, 372950}, // Married jointly
// -or qualifying widow(er)
{8350, 33950, 68525, 104425, 186475}, // Married separately
{11950, 45500, 117450, 190200, 372950} // Head of household
};
Suppose the taxable income is $400,000 for single filers. The tax can be computed as follows:
tax = brackets[0][0] * rates[0] +
(brackets[0][1] – brackets[0][0]) * rates[1]
(brackets[0][2] – brackets[0][1]) * rates[2]
(brackets[0][3] – brackets[0][2]) * rates[3]
(brackets[0][4] – brackets[0][3]) * rates[4]
(400000 – brackets[0][4]) * rates[5]
*8.13
+
+
+
+
(Locate the largest element) Write the following method that returns the location
of the largest element in a two-dimensional array.
public static int[] locateLargest(double[][] a)
The return value is a one-dimensional array that contains two elements. These
two elements indicate the row and column indices of the largest element in the
two-dimensional array. Write a test program that prompts the user to enter a twodimensional array and displays the location of the largest element in the array.
Here is a sample run:
Enter the number of rows and columns of the array: 3 4
Enter the array:
23.5 35 2 10
4.5 3 45 3.5
35 44 5.5 9.6
The location of the largest element is at (1, 2)
**8.14
(Explore matrix) Write a program that prompts the user to enter the length of a
square matrix, randomly fills in 0s and 1s into the matrix, prints the matrix, and
finds the rows, columns, and diagonals with all 0s or 1s. Here is a sample run of
the program:
Programming Exercises 311
Enter the size for
0111
0000
0100
1111
All 0s on row 1
All 1s on row 3
No same numbers on
No same numbers on
No same numbers on
*8.15
the matrix: 4
a column
the major diagonal
the sub-diagonal
(Geometry: same line?) Programming Exercise 6.39 gives a method for testing
whether three points are on the same line.
Write the following method to test whether all the points in the array points are
on the same line.
public static boolean sameLine(double[][] points)
Write a program that prompts the user to enter five points and displays whether
they are on the same line. Here are sample runs:
Enter five points: 3.4 2 6.5 9.5 2.3 2.3 5.5 5 -5 4
The five points are not on the same line
Enter five points: 1 1 2 2 3 3 4 4 5 5
The five points are on the same line
*8.16
(Sort two-dimensional array) Write a method to sort a two-dimensional array
using the following header:
public static void sort(int m[][])
The method performs a primary sort on rows and a secondary sort on columns.
For example, the following array
{{4, 2},{1, 7},{4, 5},{1, 2},{1, 1},{4, 1}}
will be sorted to
{{1, 1},{1, 2},{1, 7},{4, 1},{4, 2},{4, 5}}.
***8.17
(Financial tsunami) Banks lend money to each other. In tough economic times,
if a bank goes bankrupt, it may not be able to pay back the loan. A bank’s
total assets are its current balance plus its loans to other banks. The diagram in
Figure 8.8 shows five banks. The banks’ current balances are 25, 125, 175, 75,
and 181 million dollars, respectively. The directed edge from node 1 to node 2
indicates that bank 1 lends 40 million dollars to bank 2.
312 Chapter 8
Multidimensional Arrays
125
1
100.5
25
85
75
0
3
125
125
320.5
125
181
FIGURE 8.8
4
40
75
2
175
Banks lend money to each other.
If a bank’s total assets are under a certain limit, the bank is unsafe. The money it
borrowed cannot be returned to the lender, and the lender cannot count the loan in
its total assets. Consequently, the lender may also be unsafe, if its total assets are
under the limit. Write a program to find all the unsafe banks. Your program reads
the input as follows. It first reads two integers n and limit, where n indicates the
number of banks and limit is the minimum total assets for keeping a bank safe. It
then reads n lines that describe the information for n banks with IDs from 0 to n-1.
The first number in the line is the bank’s balance, the second number indicates
the number of banks that borrowed money from the bank, and the rest are pairs
of two numbers. Each pair describes a borrower. The first number in the pair
is the borrower’s ID and the second is the amount borrowed. For example, the
input for the five banks in Figure 8.8 is as follows (note that the limit is 201):
5 201
25 2 1 100.5 4 320.5
125 2 2 40 3 85
175 2 0 125 3 75
75 1 0 125
181 1 2 125
The total assets of bank 3 are (75 + 125), which is under 201, so bank 3 is
unsafe. After bank 3 becomes unsafe, the total assets of bank 1 fall below
(125 + 40). Thus, bank 1 is also unsafe. The output of the program should be
Unsafe banks are 3 1
*8.18
(Hint: Use a two-dimensional array borrowers to represent loans.
borrowers[i][j] indicates the loan that bank i loans to bank j. Once bank j
becomes unsafe, borrowers[i][j] should be set to 0.)
(Shuffle rows) Write a method that shuffles the rows in a two-dimensional int
array using the following header:
public static void shuffle(int[][] m)
Write a test program that shuffles the following matrix:
int[][] m = {{1, 2}, {3, 4}, {5, 6}, {7, 8}, {9, 10}};
**8.19
(Pattern recognition: four consecutive equal numbers) Write the following
method that tests whether a two-dimensional array has four consecutive numbers of the same value, either horizontally, vertically, or diagonally.
public static boolean isConsecutiveFour(int[][] values)
Programming Exercises 313
Write a test program that prompts the user to enter the number of rows and columns of a two-dimensional array and then the values in the array and displays
true if the array contains four consecutive numbers with the same value. Otherwise, display false. Here are some examples of the true cases:
0 1 0 3 1 6 1
0 1 0 3 1 6 1
0 1 0 3 1 6 1
0 1 0 3 1 6 1
0 1 6 8 6 0 1
0 1 6 8 6 0 1
0 1 6 8 6 0 1
0 1 6 8 6 0 1
5 6 2 1 8 2 9
5 5 2 1 8 2 9
5 6 2 1 6 2 9
9 6 2 1 8 2 9
6 5 6 1 1 9 1
6 5 6 1 1 9 1
6 5 6 6 1 9 1
6 9 6 1 1 9 1
1 3 6 1 4 0 7
1 5 6 1 4 0 7
1 3 6 1 4 0 7
1 3 9 1 4 0 7
3 3 3 3 4 0 7
3 5 3 3 4 0 7
3 6 3 3 4 0 7
3 3 3 9 4 0 7
***8.20
(Game: connect four) Connect four is a two-player board game in which the
players alternately drop colored disks into a seven-column, six-row vertically
suspended grid, as shown below.
The objective of the game is to connect four same-colored disks in a row, a column, or a diagonal before your opponent can do likewise. The program prompts
two players to drop a red or yellow disk alternately. In the preceding figure, the
red disk is shown in a dark color and the yellow in a light color. Whenever a disk
is dropped, the program redisplays the board on the console and determines the
status of the game (win, draw, or continue). Here is a sample run:
| | | | | | | |
| | | | | | | |
| | | | | | | |
| | | | | | | |
| | | | | | | |
| | | | | | | |
———————————————
Drop a red disk at column (0–6): 0
| | | | | | | |
| | | | | | | |
| | | | | | | |
| | | | | | | |
| | | | | | | |
|R| | | | | | |
———————————————
314 Chapter 8
Multidimensional Arrays
Drop a yellow disk at column (0–6): 3
| |
| |
| |
| |
| |
|R|
|
|
|
|
|
|
| |
| |
| |
| |
| |
|Y|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
. . .
. . .
. . .
Drop a yellow disk at column (0–6): 6
| | | | | | | |
| | | | | | | |
| | | |R| | | |
| | | |Y|R|Y| |
| | |R|Y|Y|Y|Y|
|R|Y|R|Y|R|R|R|
———————————————
The yellow player won
*8.21
(Central city) Given a set of cities, the central city is the city that has the shortest
total distance to all other cities. Write a program that prompts the user to enter
the number of the cities and the locations of the cities (coordinates), and finds
the central city and its total distance to all other cities.
Enter the number of cities: 5
Enter the coordinates of the cities:
2.5 5 5.1 3 1 9 5.4 54 5.5 2.1
The central city is at (2.5, 5.0)
The total distance to all other cities is 60.81
*8.22
VideoNote
Even number of 1s
*8.23
(Even number of 1s) Write a program that generates a 6-by-6 two-dimensional
matrix filled with 0s and 1s, displays the matrix, and checks if every row and
every column have an even number of 1s.
(Game: find the flipped cell) Suppose you are given a 6-by-6 matrix filled with
0s and 1s. All rows and all columns have an even number of 1s. Let the user flip
one cell (i.e., flip from 1 to 0 or from 0 to 1) and write a program to find which
cell was flipped. Your program should prompt the user to enter a 6-by-6 array
with 0s and 1s and find the first row r and first column c where the even number
of the 1s property is violated (i.e., the number of 1s is not even). The flipped cell
is at (r, c). Here is a sample run:
Enter a 6-by-6 matrix row by row:
1 1 1 0 1 1
1 1 1 1 0 0
0 1 0 1 1 1
1 1 1 1 1 1
0 1 1 1 1 0
1 0 0 0 0 1
The flipped cell is at (0, 1)
Programming Exercises 315
*8.24
*8.25
(Check Sudoku solution) Listing 8.4 checks whether a solution is valid by checking whether every number is valid in the board. Rewrite the program by checking
whether every row, every column, and every small box has the numbers 1 to 9.
(Markov matrix) An n * n matrix is called a positive Markov matrix if each
element is positive and the sum of the elements in each column is 1. Write the
following method to check whether a matrix is a Markov matrix.
public static boolean isMarkovMatrix(double[][] m)
Write a test program that prompts the user to enter a 3 * 3 matrix of double
values and tests whether it is a Markov matrix. Here are sample runs:
Enter a 3-by-3 matrix row by row:
0.15 0.875 0.375
0.55 0.005 0.225
0.30 0.12 0.4
It is a Markov matrix
Enter a 3-by-3 matrix row by row:
0.95 -0.875 0.375
0.65 0.005 0.225
0.30 0.22 -0.4
It is not a Markov matrix
*8.26
(Row sorting) Implement the following method to sort the rows in a twodimensional array. A new array is returned and the original array is intact.
public static double[][] sortRows(double[][] m)
Write a test program that prompts the user to enter a 3 * 3 matrix of double
values and displays a new row-sorted matrix. Here is a sample run:
Enter a 3-by-3 matrix row by row:
0.15 0.875 0.375
0.55 0.005 0.225
0.30 0.12 0.4
The row-sorted array is
0.15 0.375 0.875
0.005 0.225 0.55
0.12 0.30 0.4
*8.27
(Column sorting) Implement the following method to sort the columns in a twodimensional array. A new array is returned and the original array is intact.
public static double[][] sortColumns(double[][] m)
316 Chapter 8
Multidimensional Arrays
Write a test program that prompts the user to enter a 3 * 3 matrix of double
values and displays a new column-sorted matrix. Here is a sample run:
Enter a 3-by-3 matrix row by row:
0.15 0.875 0.375
0.55 0.005 0.225
0.30 0.12 0.4
The column-sorted array is
0.15 0.0050 0.225
0.3 0.12
0.375
0.55 0.875 0.4
8.28
(Strictly identical arrays) The two-dimensional arrays m1 and m2 are strictly
identical if their corresponding elements are equal. Write a method that returns
true if m1 and m2 are strictly identical, using the following header:
public static boolean equals(int[][] m1, int[][] m2)
Write a test program that prompts the user to enter two 3 * 3 arrays of
integers and displays whether the two are strictly identical. Here are the
sample runs.
Enter list1: 51 22 25 6 1 4 24 54 6
Enter list2: 51 22 25 6 1 4 24 54 6
The two arrays are strictly identical
Enter list1: 51 25 22 6 1 4 24 54 6
Enter list2: 51 22 25 6 1 4 24 54 6
The two arrays are not strictly identical
8.29
(Identical arrays) The two-dimensional arrays m1 and m2 are identical if they
have the same contents. Write a method that returns true if m1 and m2 are identical, using the following header:
public static boolean equals(int[][] m1, int[][] m2)
Write a test program that prompts the user to enter two 3 * 3 arrays of integers
and displays whether the two are identical. Here are the sample runs.
Enter list1: 51 25 22 6 1 4 24 54 6
Enter list2: 51 22 25 6 1 4 24 54 6
The two arrays are identical
Enter list1: 51 5 22 6 1 4 24 54 6
Enter list2: 51 22 25 6 1 4 24 54 6
The two arrays are not identical
Programming Exercises 317
*8.30
(Algebra: solve linear equations) Write a method that solves the following
2 * 2 system of linear equations:
a00x + a01y = b0
a10x + a11y = b1
x =
b0a11 - b1a01
a00a11 - a01a10
y =
b1a00 - b0a10
a00a11 - a01a10
The method header is
public static double[] linearEquation(double[][] a, double[] b)
*8.31
The method returns null if a00a11 - a01a10 is 0. Write a test program that
prompts the user to enter a00, a01, a10, a11, b0, and b1, and displays the result. If
a00a11 - a01a10 is 0, report that “The equation has no solution.” A sample run is
similar to Programming Exercise 3.3.
(Geometry: intersecting point) Write a method that returns the intersecting point of
two lines. The intersecting point of the two lines can be found by using the formula
shown in Programming Exercise 3.25. Assume that (x1, y1) and (x2, y2) are the
two points on line 1 and (x3, y3) and (x4, y4) are on line 2. The method header is
public static double[] getIntersectingPoint(double[][] points)
*8.32
The points are stored in a 4-by-2 two-dimensional array points with
(points[0][0], points[0][1]) for (x1, y1). The method returns the intersecting point or null if the two lines are parallel. Write a program that prompts
the user to enter four points and displays the intersecting point. See Programming Exercise 3.25 for a sample run.
(Geometry: area of a triangle) Write a method that returns the area of a triangle
using the following header:
public static double getTriangleArea(double[][] points)
The points are stored in a 3-by-2 two-dimensional array points with points[0]
[0] and points[0][1] for (x1, y1). The triangle area can be computed using the
formula in Programming Exercise 2.19. The method returns 0 if the three points
are on the same line. Write a program that prompts the user to enter three points of
a triangle and displays the triangle's area. Here is a sample run of the program:
Enter x1, y1, x2, y2, x3, y3: 2.5 2 5 -1.0 4.0 2.0
The area of the triangle is 2.25
Enter x1, y1, x2, y2, x3, y3: 2 2 4.5 4.5 6 6
The three points are on the same line
*8.33
(Geometry: polygon subareas) A convex 4-vertex polygon is divided into four
triangles, as shown in Figure 8.9.
Write a program that prompts the user to enter the coordinates of four vertices and
displays the areas of the four triangles in increasing order. Here is a sample run:
Enter x1, y1, x2, y2, x3, y3, x4, y4:
-2.5 2 4 4 3 -2 -2 -3.5
The areas are 6.17 7.96 8.08 10.42
318 Chapter 8
Multidimensional Arrays
v2 (x2, y2)
v1 (x1, y1)
v3 (x3, y3)
v4 (x4, y4)
FIGURE 8.9
*8.34
A 4-vertex polygon is defined by four vertices.
(Geometry: rightmost lowest point) In computational geometry, often you need
to find the rightmost lowest point in a set of points. Write the following method
that returns the rightmost lowest point in a set of points.
public static double[]
getRightmostLowestPoint(double[][] points)
Write a test program that prompts the user to enter the coordinates of six points
and displays the rightmost lowest point. Here is a sample run:
Enter 6 points: 1.5 2.5 -3 4.5 5.6 -7 6.5 -7 8 1 10 2.5
The rightmost lowest point is (6.5, -7.0)
**8.35
(Largest block) Given a square matrix with the elements 0 or 1, write a program
to find a maximum square submatrix whose elements are all 1s. Your program
should prompt the user to enter the number of rows in the matrix. The program
then displays the location of the first element in the maximum square submatrix
and the number of the rows in the submatrix. Here is a sample run:
Enter the number of rows in the matrix: 5
Enter the matrix row by row:
1 0 1 0 1
1 1 1 0 1
1 0 1 1 1
1 0 1 1 1
1 0 1 1 1
The maximum square submatrix is at (2, 2) with size 3
Your program should implement and use the following method to find the maximum square submatrix:
public static int[] findLargestBlock(int[][] m)
The return value is an array that consists of three values. The first two values are
the row and column indices for the first element in the submatrix, and the third
value is the number of the rows in the submatrix.
**8.36
(Latin square) A Latin square is an n-by-n array filled with n different Latin letters, each occurring exactly once in each row and once in each column. Write a
Programming Exercises 319
program that prompts the user to enter the number n and the array of characters,
as shown in the sample output, and checks if the input array is a Latin square.
The characters are the first n characters starting from A.
Enter number n: 4
Enter 4 rows of letters separated by spaces:
A B C D
B A D C
C D B A
D C A B
The input array is a Latin square
Enter number n: 3
Enter 3 rows of letters separated by spaces:
A F D
Wrong input: the letters must be from A to C
**8.37
(Guess the capitals) Write a program that repeatedly prompts the user to enter
a capital for a state. Upon receiving the user input, the program reports whether
the answer is correct. Assume that 50 states and their capitals are stored in a twodimensional array, as shown in Figure 8.10. The program prompts the user to
answer all states’ capitals and displays the total correct count. The user’s answer
is not case-sensitive.
Alabama
Alaska
Arizona
...
...
Montgomery
Juneau
Phoenix
...
...
FIGURE 8.10 A two-dimensional array stores states and their capitals.
Here is a sample run:
What is the
The correct
What is the
Your answer
What is the
...
The correct
capital of Alabama? Montogomery
answer should be Montgomery
capital of Alaska? Juneau
is correct
capital of Arizona? ...
count is 35
This page intentionally left blank
CHAPTER
OBJECTS AND CLASSES
Objectives
■
To describe objects and classes, and use classes to model objects (§9.2).
■
To use UML graphical notation to describe classes and objects (§9.2).
■
To demonstrate how to define classes and create objects (§9.3).
■
To create objects using constructors (§9.4).
■
To access objects via object reference variables (§9.5).
■
To define a reference variable using a reference type (§9.5.1).
■
To access an object’s data and methods using the object member access
operator (.) (§9.5.2).
■
To define data fields of reference types and assign default values for an
object’s data fields (§9.5.3).
■
To distinguish between object reference variables and primitive data
type variables (§9.5.4).
■
To use the Java library classes Date, Random, and Point2D (§9.6).
■
To distinguish between instance and static variables and methods (§9.7).
■
To define private data fields with appropriate getter and setter methods
(§9.8).
■
To encapsulate data fields to make classes easy to maintain (§9.9).
■
To develop methods with object arguments and differentiate between
primitive-type arguments and object-type arguments (§9.10).
■
To store and process objects in arrays (§9.11).
■
To create immutable objects from immutable classes to protect the
contents of objects (§9.12).
■
To determine the scope of variables in the context of a class (§9.13).
■
To use the keyword this to refer to the calling object itself (§9.14).
9
322 Chapter 9
Objects and Classes
9.1 Introduction
Key
Point
Object-oriented programming enables you to develop large-scale software and GUIs
effectively.
Having learned the material in the preceding chapters, you are able to solve many programming problems using selections, loops, methods, and arrays. However, these Java features
are not sufficient for developing graphical user interfaces and large-scale software systems.
Suppose you want to develop a graphical user interface (GUI, pronounced goo-ee) as shown
in Figure 9.1. How would you program it?
why OOP?
Button
Label
FIGURE 9.1
Text Field Check Box
Radio Button
Combo Box
The GUI objects are created from classes.
This chapter introduces object-oriented programming, which you can use to develop GUI
and large-scale software systems.
9.2 Defining Classes for Objects
Key
Point
VideoNote
Define classes and objects
object
state of an object
properties
attributes
data fields
behavior
actions
class
contract
instantiation
instance
data field
method
constructors
A class defines the properties and behaviors for objects.
Object-oriented programming (OOP) involves programming using objects. An object represents an entity in the real world that can be distinctly identified. For example, a student, a
desk, a circle, a button, and even a loan can all be viewed as objects. An object has a unique
identity, state, and behavior.
■
The state of an object (also known as its properties or attributes) is represented by
data fields with their current values. A circle object, for example, has a data field
radius, which is the property that characterizes a circle. A rectangle object has the
data fields width and height, which are the properties that characterize a rectangle.
■
The behavior of an object (also known as its actions) is defined by methods. To
invoke a method on an object is to ask the object to perform an action. For example, you may define methods named getArea() and getPerimeter() for circle
objects. A circle object may invoke getArea() to return its area and getPerimeter() to return its perimeter. You may also define the setRadius(radius)
method. A circle object can invoke this method to change its radius.
Objects of the same type are defined using a common class. A class is a template, blueprint, or contract that defines what an object’s data fields and methods will be. An object is an
instance of a class. You can create many instances of a class. Creating an instance is referred
to as instantiation. The terms object and instance are often interchangeable. The relationship
between classes and objects is analogous to that between an apple-pie recipe and apple pies:
You can make as many apple pies as you want from a single recipe. Figure 9.2 shows a class
named Circle and its three objects.
A Java class uses variables to define data fields and methods to define actions. Additionally, a class provides methods of a special type, known as constructors, which are invoked to
create a new object. A constructor can perform any action, but constructors are designed to
perform initializing actions, such as initializing the data fields of objects. Figure 9.3 shows an
example of defining the class for circle objects.
9.2 Defining Classes for Objects 323
A class template
Class Name: Circle
Data Fields:
radius is _____
Methods:
getArea
getPerimeter
setRadius
Circle Object 1
Circle Object 2
Circle Object 3
Data Fields:
radius is 1
Data Fields:
radius is 25
Data Fields:
radius is 125
Three objects of
the Circle class
FIGURE 9.2 A class is a template for creating objects.
class Circle {
/** The radius of this circle */
double radius = 1;
Data field
/** Construct a circle object */
Circle() {
}
Constructors
/** Construct a circle object */
Circle(double newRadius) {
radius = newRadius;
}
/** Return the area of this circle */
double getArea() {
return radius * radius * Math.PI;
}
/** Return the perimeter of this circle */
double getPerimeter() {
return 2 * radius * Math.PI;
}
Method
/** Set new radius for this circle */
double setRadius(double newRadius) {
radius = newRadius;
}
}
FIGURE 9.3 A class is a construct that defines objects of the same type.
The Circle class is different from all of the other classes you have seen thus far. It does
not have a main method and therefore cannot be run; it is merely a definition for circle objects.
The class that contains the main method will be referred to in this book, for convenience, as
the main class.
The illustration of class templates and objects in Figure 9.2 can be standardized using Unified
Modeling Language (UML) notation. This notation, as shown in Figure 9.4, is called a UML
class diagram, or simply a class diagram. In the class diagram, the data field is denoted as
dataFieldName: dataFieldType
The constructor is denoted as
ClassName(parameterName: parameterType)
main class
Unified Modeling Language
(UML)
class diagram
324 Chapter 9
Objects and Classes
UML Class Diagram
Class name
Circle
radius: double
Data fields
Circle()
Constructors and
methods
Circle(newRadius: double)
getArea(): double
getPerimeter(): double
setRadius(newRadius: double): void
FIGURE 9.4
circle1: Circle
circle2: Circle
circle3: Circle
radius = 1
radius = 25
radius = 125
UML notation
for objects
Classes and objects can be represented using UML notation.
The method is denoted as
methodName(parameterName: parameterType): returnType
9.3 Example: Defining Classes and Creating Objects
Key
Point
Classes are definitions for objects and objects are created from classes.
This section gives two examples of defining classes and uses the classes to create objects.
Listing 9.1 is a program that defines the Circle class and uses it to create objects. The program constructs three circle objects with radius 1, 25, and 125 and displays the radius and
area of each of the three circles. It then changes the radius of the second object to 100 and
displays its new radius and area.
Note
To avoid a naming conflict with several enhanced versions of the Circle class introduced later in the chapter, the Circle class in this example is named SimpleCircle.
For simplicity, we will still refer to the class in the text as Circle.
avoid naming conflicts
LISTING 9.1 TestSimpleCircle.java
main class
main method
create object
create object
create object
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
public class TestSimpleCircle {
/** Main method */
public static void main(String[] args) {
// Create a circle with radius 1
SimpleCircle circle1 = new SimpleCircle();
System.out.println("The area of the circle of radius "
+ circle1.radius + " is " + circle1.getArea());
// Create a circle with radius 25
SimpleCircle circle2 = new SimpleCircle(25);
System.out.println("The area of the circle of radius "
+ circle2.radius + " is " + circle2.getArea());
// Create a circle with radius 125
SimpleCircle circle3 = new SimpleCircle(125);
System.out.println("The area of the circle of radius "
+ circle3.radius + " is " + circle3.getArea());
// Modify circle radius
circle2.radius = 100; // or circle2.setRadius(100)
System.out.println("The area of the circle of radius "
+ circle2.radius + " is " + circle2.getArea());
9.3 Example: Defining Classes and Creating Objects 325
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
}
}
// Define the circle class with two constructors
class SimpleCircle {
double radius;
class SimpleCircle
data field
/** Construct a circle with radius 1 */
SimpleCircle() {
radius = 1;
}
no-arg constructor
/** Construct a circle with a specified radius */
SimpleCircle(double newRadius) {
radius = newRadius;
}
second constructor
/** Return the area of this circle */
double getArea() {
return radius * radius * Math.PI;
}
getArea
/** Return the perimeter of this circle */
double getPerimeter() {
return 2 * radius * Math.PI;
}
getPerimeter
/** Set a new radius for this circle */
void setRadius(double newRadius) {
radius = newRadius;
}
setRadius
}
The
The
The
The
area
area
area
area
of
of
of
of
the
the
the
the
circle
circle
circle
circle
of
of
of
of
radius
radius
radius
radius
1.0 is 3.141592653589793
25.0 is 1963.4954084936207
125.0 is 49087.385212340516
100.0 is 31415.926535897932
The program contains two classes. The first of these, TestSimpleCircle, is the main class.
Its sole purpose is to test the second class, SimpleCircle. Such a program that uses the class
is often referred to as a client of the class. When you run the program, the Java runtime system
invokes the main method in the main class.
You can put the two classes into one file, but only one class in the file can be a public class.
Furthermore, the public class must have the same name as the file name. Therefore, the file
name is TestSimpleCircle.java, since TestSimpleCircle is public. Each class in the source
code is compiled into a .class file. When you compile TestSimpleCircle.java, two class files
TestSimpleCircle.class and SimpleCircle.class are generated, as shown in Figure 9.5.
// File TestSimpleCircle.java
public class TestSimpleCircle {
…
}
class SimpleCircle {
…
}
generates
TestSimpleCircle.class
generates
SimpleCircle.class
Java
compiled Compiler
by
FIGURE 9.5 Each class in the source code file is compiled into a .class file.
client
public class
326 Chapter 9
Objects and Classes
The main class contains the main method (line 3) that creates three objects. As in creating an
array, the new operator is used to create an object from the constructor: new SimpleCircle()
creates an object with radius 1 (line 5), new SimpleCircle(25) creates an object with radius
25 (line 10), and new SimpleCircle(125) creates an object with radius 125 (line 15).
These three objects (referenced by circle1, circle2, and circle3) have different
data but the same methods. Therefore, you can compute their respective areas by using
the getArea() method. The data fields can be accessed via the reference of the object
using circle1.radius, circle2.radius, and circle3.radius, respectively. The
object can invoke its method via the reference of the object using circle1.getArea(),
circle2.getArea(), and circle3.getArea(), respectively.
These three objects are independent. The radius of circle2 is changed to 100 in line 20.
The object’s new radius and area are displayed in lines 21–22.
There are many ways to write Java programs. For instance, you can combine the two
classes in the example into one, as shown in Listing 9.2.
LISTING 9.2 SimpleCircle.java
main method
data field
no-arg constructor
second constructor
method
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
public class SimpleCircle {
/** Main method */
public static void main(String[] args) {
// Create a circle with radius 1
SimpleCircle circle1 = new SimpleCircle();
System.out.println("The area of the circle of radius "
+ circle1.radius + " is " + circle1.getArea());
// Create a circle with radius 25
SimpleCircle circle2 = new SimpleCircle(25);
System.out.println("The area of the circle of radius "
+ circle2.radius + " is " + circle2.getArea());
// Create a circle with radius 125
SimpleCircle circle3 = new SimpleCircle(125);
System.out.println("The area of the circle of radius "
+ circle3.radius + " is " + circle3.getArea());
// Modify circle radius
circle2.radius = 100;
System.out.println("The area of the circle of radius "
+ circle2.radius + " is " + circle2.getArea());
}
double radius;
/** Construct a circle with radius 1 */
SimpleCircle() {
radius = 1;
}
/** Construct a circle with a specified radius */
SimpleCircle(double newRadius) {
radius = newRadius;
}
/** Return the area of this circle */
double getArea() {
return radius * radius * Math.PI;
}
9.3 Example: Defining Classes and Creating Objects 327
42
43
44
45
46
47
48
49
50
51
/** Return the perimeter of this circle */
double getPerimeter() {
return 2 * radius * Math.PI;
}
/** Set a new radius for this circle */
void setRadius(double newRadius) {
radius = newRadius;
}
}
Since the combined class has a main method, it can be executed by the Java interpreter. The
main method is the same as that in Listing 9.1. This demonstrates that you can test a class by
simply adding a main method in the same class.
As another example, consider television sets. Each TV is an object with states (current
channel, current volume level, power on or off) and behaviors (change channels, adjust volume, turn on/off). You can use a class to model TV sets. The UML diagram for the class is
shown in Figure 9.6.
TV
channel: int
volumeLevel: int
The + sign indicates
public modifier
The current channel (1 to 120) of this TV.
on: boolean
The current volume level (1 to 7) of this TV.
Indicates whether this TV is on/off.
+TV()
+turnOn(): void
+turnOff(): void
Constructs a default TV object.
Turns on this TV.
Turns off this TV.
+setChannel(newChannel: int): void
+setVolume(newVolumeLevel: int): void
+channelUp(): void
Sets a new channel for this TV.
Sets a new volume level for this TV.
Increases the channel number by 1.
Decreases the channel number by 1.
+channelDown(): void
+volumeUp(): void
+volumeDown(): void
Increases the volume level by 1.
Decreases the volume level by 1.
FIGURE 9.6 The TV class models TV sets.
Listing 9.3 gives a program that defines the TV class.
LISTING 9.3 TV.java
1
2
3
4
5
6
7
8
9
10
11
12
13
public class TV {
int channel = 1; // Default channel is 1
int volumeLevel = 1; // Default volume level is 1
boolean on = false; // TV is off
data fields
public TV() {
}
constructor
public void turnOn() {
on = true;
}
turn on TV
public void turnOff() {
turn off TV
328 Chapter 9
set a new channel
set a new volume
increase channel
decrease channel
increase volume
decrease volume
Objects and Classes
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
on = false;
}
public void setChannel(int newChannel) {
if (on && newChannel >= 1 && newChannel <= 120)
channel = newChannel;
}
public void setVolume(int newVolumeLevel) {
if (on && newVolumeLevel >= 1 && newVolumeLevel <= 7)
volumeLevel = newVolumeLevel;
}
public void channelUp() {
if (on && channel < 120)
channel++;
}
public void channelDown() {
if (on && channel > 1)
channel—–;
}
public void volumeUp() {
if (on && volumeLevel < 7)
volumeLevel++;
}
public void volumeDown() {
if (on && volumeLevel > 1)
volumeLevel—–;
}
}
The constructor and methods in the TV class are defined public so they can be accessed from
other classes. Note that the channel and volume level are not changed if the TV is not on. Before
either of these is changed, its current value is checked to ensure that it is within the correct range.
Listing 9.4 gives a program that uses the TV class to create two objects.
LISTING 9.4 TestTV.java
main method
create a TV
turn on
set a new channel
set a new volume
create a TV
turn on
increase channel
increase volume
display state
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
public class TestTV {
public static void main(String[] args) {
TV tv1 = new TV();
tv1.turnOn();
tv1.setChannel(30);
tv1.setVolume(3);
TV tv2 = new TV();
tv2.turnOn();
tv2.channelUp();
tv2.channelUp();
tv2.volumeUp();
System.out.println("tv1's
+ " and volume level is
System.out.println("tv2's
+ " and volume level is
}
}
channel is " + tv1.channel
" + tv1.volumeLevel);
channel is " + tv2.channel
" + tv2.volumeLevel);
9.4 Constructing Objects Using Constructors 329
tv1's channel is 30 and volume level is 3
tv2's channel is 3 and volume level is 2
The program creates two objects in lines 3 and 8 and invokes the methods on the objects to
perform actions for setting channels and volume levels and for increasing channels and volumes. The program displays the state of the objects in lines 14–17. The methods are invoked
using syntax such as tv1.turnOn() (line 4). The data fields are accessed using syntax such
as tv1.channel (line 14).
These examples have given you a glimpse of classes and objects. You may have many
questions regarding constructors, objects, reference variables, accessing data fields, and
invoking object’s methods. The sections that follow discuss these issues in detail.
9.1
9.2
9.3
9.4
Describe the relationship between an object and its defining class.
How do you define a class?
How do you declare an object’s reference variable?
How do you create an object?
✓
Check
Point
9.4 Constructing Objects Using Constructors
A constructor is invoked to create an object using the new operator.
Constructors are a special kind of method. They have three peculiarities:
Key
Point
■
A constructor must have the same name as the class itself.
constructor’s name
■
Constructors do not have a return type—not even void.
no return type
■
Constructors are invoked using the new operator when an object is created.
Constructors play the role of initializing objects.
new operator
The constructor has exactly the same name as its defining class. Like regular methods,
constructors can be overloaded (i.e., multiple constructors can have the same name but different signatures), making it easy to construct objects with different initial data values.
It is a common mistake to put the void keyword in front of a constructor. For example,
public void Circle() {
}
In this case, Circle() is a method, not a constructor.
Constructors are used to construct objects. To construct an object from a class, invoke a
constructor of the class using the new operator, as follows:
overloaded constructors
no void
constructing objects
new ClassName(arguments);
For example, new Circle() creates an object of the Circle class using the first constructor defined in the Circle class, and new Circle(25) creates an object using the second
constructor defined in the Circle class.
A class normally provides a constructor without arguments (e.g., Circle()). Such a constructor is referred to as a no-arg or no-argument constructor.
A class may be defined without constructors. In this case, a public no-arg constructor with
an empty body is implicitly defined in the class. This constructor, called a default constructor,
is provided automatically only if no constructors are explicitly defined in the class.
9.5
9.6
What are the differences between constructors and methods?
When will a class have a default constructor?
no-arg constructor
default constructor
✓
Check
Point
330 Chapter 9
Objects and Classes
9.5 Accessing Objects via Reference Variables
Key
Point
An object’s data and methods can be accessed through the dot (.) operator via the
object’s reference variable.
Newly created objects are allocated in the memory. They can be accessed via reference
variables.
9.5.1
reference variable
Reference Variables and Reference Types
Objects are accessed via the object’s reference variables, which contain references to the
objects. Such variables are declared using the following syntax:
ClassName objectRefVar;
reference type
A class is essentially a programmer-defined type. A class is a reference type, which means
that a variable of the class type can reference an instance of the class. The following statement
declares the variable myCircle to be of the Circle type:
Circle myCircle;
The variable myCircle can reference a Circle object. The next statement creates an object
and assigns its reference to myCircle:
myCircle = new Circle();
You can write a single statement that combines the declaration of an object reference variable,
the creation of an object, and the assigning of an object reference to the variable with the following syntax:
ClassName objectRefVar = new ClassName();
Here is an example:
Circle myCircle = new Circle();
The variable myCircle holds a reference to a Circle object.
Note
An object reference variable that appears to hold an object actually contains a reference
to that object. Strictly speaking, an object reference variable and an object are different,
but most of the time the distinction can be ignored. Therefore, it is fine, for simplicity, to
say that myCircle is a Circle object rather than use the longer-winded description
that myCircle is a variable that contains a reference to a Circle object.
object vs. object reference
variable
Note
Arrays are treated as objects in Java. Arrays are created using the new operator. An array
variable is actually a variable that contains a reference to an array.
array object
9.5.2
dot operator (.)
Accessing an Object’s Data and Methods
In OOP terminology, an object’s member refers to its data fields and methods. After an object
is created, its data can be accessed and its methods can be invoked using the dot operator (.),
also known as the object member access operator:
■ objectRefVar.dataField
references a data field in the object.
■ objectRefVar.method(arguments)
invokes a method on the object.
9.5 Accessing Objects via Reference Variables 331
For example, myCircle.radius references the radius in myCircle, and myCircle
.getArea() invokes the getArea method on myCircle. Methods are invoked as operations
on objects.
The data field radius is referred to as an instance variable, because it is dependent on
a specific instance. For the same reason, the method getArea is referred to as an instance
method, because you can invoke it only on a specific instance. The object on which an instance
method is invoked is called a calling object.
instance variable
instance method
calling object
Caution
Recall that you use Math.methodName(arguments) (e.g., Math.pow(3, 2.5))
to invoke a method in the Math class. Can you invoke getArea() using
Circle.getArea()? The answer is no. All the methods in the Math class are static
methods, which are defined using the static keyword. However, getArea() is
an instance method, and thus nonstatic. It must be invoked from an object using
objectRefVar.methodName(arguments) (e.g., myCircle.getArea()).
Further explanation is given in Section 9.7, Static Variables, Constants, and Methods.
invoking methods
Note
Usually you create an object and assign it to a variable, and then later you can use the
variable to reference the object. Occasionally an object does not need to be referenced
later. In this case, you can create an object without explicitly assigning it to a variable
using the syntax:
new Circle();
or
System.out.println("Area is " + new Circle(5).getArea());
The former statement creates a Circle object. The latter creates a Circle object and
invokes its getArea method to return its area. An object created in this way is known
as an anonymous object.
9.5.3
anonymous object
Reference Data Fields and the null Value
The data fields can be of reference types. For example, the following Student class contains
a data field name of the String type. String is a predefined Java class.
reference data fields
class Student {
String name; // name has the default value null
int age; // age has the default value 0
boolean isScienceMajor; // isScienceMajor has default value false
char gender; // gender has default value '\u0000'
}
If a data field of a reference type does not reference any object, the data field holds a special
Java value, null. null is a literal just like true and false. While true and false are
Boolean literals, null is a literal for a reference type.
The default value of a data field is null for a reference type, 0 for a numeric type, false
for a boolean type, and \u0000 for a char type. However, Java assigns no default value to
a local variable inside a method. The following code displays the default values of the data
fields name, age, isScienceMajor, and gender for a Student object:
class Test {
public static void main(String[] args) {
Student student = new Student();
System.out.println("name? " + student.name);
null value
default field values
332 Chapter 9
Objects and Classes
System.out.println("age? " + student.age);
System.out.println("isScienceMajor? " + student.isScienceMajor);
System.out.println("gender? " + student.gender);
}
}
The following code has a compile error, because the local variables x and y are not initialized:
class Test {
public static void main(String[] args) {
int x; // x has no default value
String y; // y has no default value
System.out.println("x is " + x);
System.out.println("y is " + y);
}
}
Caution
NullPointerException is a common runtime error. It occurs when you invoke
a method on a reference variable with a null value. Make sure you assign an object
NullPointerException
reference to the variable before invoking the method through the reference variable (See
Checkpoint Question 9.11c).
9.5.4
Differences between Variables of Primitive Types
and Reference Types
Every variable represents a memory location that holds a value. When you declare a variable,
you are telling the compiler what type of value the variable can hold. For a variable of a primitive type, the value is of the primitive type. For a variable of a reference type, the value is a
reference to where an object is located. For example, as shown in Figure 9.7, the value of int
variable i is int value 1, and the value of Circle object c holds a reference to where the
contents of the Circle object are stored in memory.
When you assign one variable to another, the other variable is set to the same value. For
a variable of a primitive type, the real value of one variable is assigned to the other variable.
For a variable of a reference type, the reference of one variable is assigned to the other variable. As shown in Figure 9.8, the assignment statement i = j copies the contents of j into i
Created using new Circle()
Primitive type
int i = 1
Object type
Circle c c
1
i
c: Circle
reference
radius = 1
FIGURE 9.7 A variable of a primitive type holds a value of the primitive type, and a variable
of a reference type holds a reference to where an object is stored in memory.
Primitive type assignment i = j
Before:
FIGURE 9.8
After:
i
1
i
2
j
2
j
2
Primitive variable j is copied to variable i.
9.5 Accessing Objects via Reference Variables 333
for primitive variables. As shown in Figure 9.9, the assignment statement c1 = c2 copies
the reference of c2 into c1 for reference variables. After the assignment, variables c1 and c2
refer to the same object.
Object type assignment c1 = c2
Before:
After:
c1
c1
c2
c2
c2: Circle
c1: Circle
c2: Circle
c1: Circle
radius = 9
radius = 5
radius = 9
radius = 5
FIGURE 9.9 Reference variable c2 is copied to variable c1.
Note
As illustrated in Figure 9.9, after the assignment statement c1 = c2, c1 points to the
same object referenced by c2. The object previously referenced by c1 is no longer useful
and therefore is now known as garbage. Garbage occupies memory space, so the Java
runtime system detects garbage and automatically reclaims the space it occupies. This
process is called garbage collection.
garbage
garbage collection
Tip
If you know that an object is no longer needed, you can explicitly assign null to a reference variable for the object. The JVM will automatically collect the space if the object is
not referenced by any reference variable.
9.7
9.8
9.9
9.10
9.11
1
2
3
4
5
Check
Point
What is an anonymous object?
What is NullPointerException?
Is an array an object or a primitive type value? Can an array contain elements of an
object type? Describe the default value for the elements of an array.
What is wrong with each of the following programs?
public class ShowErrors {
public static void main(String[] args) {
ShowErrors t = new ShowErrors(5);
}
}
1
2
3
4
5
6
public class ShowErrors {
public static void main(String[] args) {
ShowErrors t = new ShowErrors();
t.x();
}
}
(a)
1
2
3
4
5
6
7
8
✓
Which operator is used to access a data field or invoke a method from an object?
public class ShowErrors {
public void method1() {
Circle c;
System.out.println("What is radius "
+ c.getRadius());
c = new Circle();
}
}
(c)
(b)
1
2
3
4
5
6
7
8
9
10
public class ShowErrors {
public static void main(String[] args) {
C c = new C(5.0);
System.out.println(c.value);
}
}
class C {
int value = 2;
}
(d)
334 Chapter 9
Objects and Classes
9.12
What is wrong in the following code?
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
9.13
class Test {
public static void main(String[] args) {
A a = new A();
a.print();
}
}
class A {
String s;
A(String newS) {
s = newS;
}
public void print() {
System.out.print(s);
}
}
What is the output of the following code?
public class A {
boolean x;
public static void main(String[] args) {
A a = new A();
System.out.println(a.x);
}
}
9.6 Using Classes from the Java Library
Key
Point
VideoNote
The Java API contains a rich set of classes for developing Java programs.
Listing 9.1 defined the SimpleCircle class and created objects from the class. You will
frequently use the classes in the Java library to develop programs. This section gives some
examples of the classes in the Java library.
9.6.1
Use classes
java.util.Date class
The Date Class
In Listing 2.7, ShowCurrentTime.java, you learned how to obtain the current time using
System.currentTimeMillis(). You used the division and remainder operators to extract
the current second, minute, and hour. Java provides a system-independent encapsulation of
date and time in the java.util.Date class, as shown in Figure 9.10.
java.util.Date
+Date()
Constructs a Date object for the current time.
+Date(elapseTime: long)
+toString(): String
Constructs a Date object for a given time in
milliseconds elapsed since January 1, 1970, GMT.
Returns a string representing the date and time.
+getTime(): long
Returns the number of milliseconds since January 1,
+setTime(elapseTime: long): void
1970, GMT.
Sets a new elapse time in the object.
FIGURE 9.10 A Date object represents a specific date and time.
9.6 Using Classes from the Java Library 335
You can use the no-arg constructor in the Date class to create an instance for the current date and time, the getTime() method to return the elapsed time since January 1, 1970,
GMT, and the toString() method to return the date and time as a string. For example, the
following code
java.util.Date date = new java.util.Date();
System.out.println("The elapsed time since Jan 1, 1970 is " +
date.getTime() + " milliseconds");
System.out.println(date.toString());
create object
get elapsed time
invoke toString
displays the output like this:
The elapsed time since Jan 1, 1970 is 1324903419651 milliseconds
Mon Dec 26 07:43:39 EST 2011
The Date class has another constructor, Date(long elapseTime), which can be used to
construct a Date object for a given time in milliseconds elapsed since January 1, 1970, GMT.
9.6.2
The Random Class
You have used Math.random() to obtain a random double value between 0.0 and 1.0
(excluding 1.0). Another way to generate random numbers is to use the java.util.Random
class, as shown in Figure 9.11, which can generate a random int, long, double, float, and
boolean value.
java.util.Random
+Random()
Constructs a Random object with the current time as its seed.
+Random(seed: long)
Constructs a Random object with a specified seed.
+nextInt(): int
Returns a random int value.
+nextInt(n: int): int
Returns a random int value between 0 and n (excluding n).
+nextLong(): long
Returns a random long value.
+nextDouble(): double
Returns a random double value between 0.0 and 1.0 (excluding 1.0).
+nextFloat(): float
Returns a random float value between 0.0F and 1.0F (excluding 1.0F).
+nextBoolean(): boolean
Returns a random boolean value.
FIGURE 9.11 A Random object can be used to generate random values.
When you create a Random object, you have to specify a seed or use the default seed. A
seed is a number used to initialize a random number generator. The no-arg constructor creates a Random object using the current elapsed time as its seed. If two Random objects have
the same seed, they will generate identical sequences of numbers. For example, the following
code creates two Random objects with the same seed, 3.
Random random1 = new Random(3);
System.out.print("From random1: ");
for (int i = 0; i < 10; i++)
System.out.print(random1.nextInt(1000) + " ");
Random random2 = new Random(3);
System.out.print("\nFrom random2: ");
for (int i = 0; i < 10; i++)
System.out.print(random2.nextInt(1000) + " ");
336 Chapter 9
Objects and Classes
The code generates the same sequence of random int values:
From random1: 734 660 210 581 128 202 549 564 459 961
From random2: 734 660 210 581 128 202 549 564 459 961
Note
The ability to generate the same sequence of random values is useful in software testing
and many other applications. In software testing, often you need to reproduce the test
cases from a fixed sequence of random numbers.
same sequence
9.6.3
The Point2D Class
Java API has a conveninent Point2D class in the javafx.geometry package for representing a point in a two-dimensional plane. The UML diagram for the class is shown in Figure 9.12.
javafx.geometry.Point2D
+Point2D(x: double, y: double)
+distance(x: double, y: double): double
+distance(p: Point2D): double
+getX(): double
+getY(): double
+toString(): String
Constructs a Point2D object with the specified x- and y-coordinates.
Returns the distance between this point and the specified point (x, y).
Returns the distance between this point and the specified point p.
Returns the x-coordinate from this point.
Returns the y-coordinate from this point.
Returns a string representation for the point.
FIGURE 9.12 A Point2D object represents a point with x- and y-coordinates.
You can create a Point2D object for a point with the specified x- and y-coordinates, use
the distance method to compute the distance from this point to another point, and use the
toString() method to return a string representation of the point. Lisitng 9.5 gives an example of using this class.
LISTING 9.5 TestPoint2D.java
create an object
invoke toString()
get distance
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
import java.util.Scanner;
import javafx.geometry.Point2D;
public class TestPoint2D {
public static void main(String[] args) {
Scanner input = new Scanner(System.in);
System.out.print("Enter point1's x-, y-coordinates: ");
double x1 = input.nextDouble();
double y1 = input.nextDouble();
System.out.print("Enter point2's x-, y-coordinates: ");
double x2 = input.nextDouble();
double y2 = input.nextDouble();
Point2D p1 = new Point2D(x1, y1);
Point2D p2 = new Point2D(x2, y2);
System.out.println("p1 is " + p1.toString());
System.out.println("p2 is " + p2.toString());
System.out.println("The distance between p1 and p2 is " +
p1.distance(p2));
}
}
9.7 Static Variables, Constants, and Methods 337
Enter point1's x-, y-coordinates: 1.5 5.5
Enter point2's x-, y-coordinates: -5.3 -4.4
p1 is Point2D [x = 1.5, y = 5.5]
p2 is Point2D [x = -5.3, y = -4.4]
The distance between p1 and p2 is 12.010412149464313
This program creates two objects of the Point2D class (lines 15–16). The toString()
method returns a string that describes the object (lines 17–18). Invoking p1.distance(p2)
returns the distance between the two points (line 20).
9.14
9.15
9.16
How do you create a Date for the current time? How do you display the current time?
How do you create a Point2D? Suppose p1 and p2 are two instances of Point2D?
How do you obtain the distance between the two points?
Which packages contain the classes Date, Random, Point2D, System, and Math?
✓
Check
Point
9.7 Static Variables, Constants, and Methods
A static variable is shared by all objects of the class. A static method cannot access
instance members of the class.
The data field radius in the circle class is known as an instance variable. An instance variable is tied to a specific instance of the class; it is not shared among objects of the same class.
For example, suppose that you create the following objects:
Circle circle1 = new Circle();
Circle circle2 = new Circle(5);
The radius in circle1 is independent of the radius in circle2 and is stored in a different memory location. Changes made to circle1’s radius do not affect circle2’s radius,
and vice versa.
If you want all the instances of a class to share data, use static variables, also known as
class variables. Static variables store values for the variables in a common memory location.
Because of this common location, if one object changes the value of a static variable, all
objects of the same class are affected. Java supports static methods as well as static variables.
Static methods can be called without creating an instance of the class.
Let’s modify the Circle class by adding a static variable numberOfObjects to count the
number of circle objects created. When the first object of this class is created, numberOfObjects is 1. When the second object is created, numberOfObjects becomes 2. The UML
of the new circle class is shown in Figure 9.13. The Circle class defines the instance variable radius and the static variable numberOfObjects, the instance methods getRadius,
setRadius, and getArea, and the static method getNumberOfObjects. (Note that static
variables and methods are underlined in the UML class diagram.)
To declare a static variable or define a static method, put the modifier static in the
variable or method declaration. The static variable numberOfObjects and the static method
getNumberOfObjects() can be declared as follows:
Key
Point
Static vs. instance
instance variable
VideoNote
Static vs. instance
static variable
static method
static int numberOfObjects;
declare static variable
static int getNumberObjects() {
return numberOfObjects;
}
define static method
338 Chapter 9
Objects and Classes
UML Notation:
underline: static variables or methods
instantiate
Circle
circle1: Circle
Memory
radius = 1
numberOfObjects = 2
1
radius
2
numberOfObjects
5
radius
radius: double
numberOfObjects: int
getNumberOfObjects(): int
getArea(): double
instantiate
After two Circle
Objects were created,
numberOfObjects
is 2.
circle2: Circle
radius = 5
numberOfObjects = 2
FIGURE 9.13 Instance variables belong to the instances and have memory storage independent of one another. Static
variables are shared by all the instances of the same class.
declare constant
Constants in a class are shared by all objects of the class. Thus, constants should be declared
as final static. For example, the constant PI in the Math class is defined as:
final static double PI = 3.14159265358979323846;
The new circle class, named CircleWithStaticMembers, is defined in Listing 9.6:
LISTING 9.6 CircleWithStaticMembers.java
static variable
increase by 1
increase by 1
static method
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
public class CircleWithStaticMembers {
/** The radius of the circle */
double radius;
/** The number of objects created */
static int numberOfObjects = 0;
/** Construct a circle with radius 1 */
CircleWithStaticMembers() {
radius = 1;
numberOfObjects++;
}
/** Construct a circle with a specified radius */
CircleWithStaticMembers(double newRadius) {
radius = newRadius;
numberOfObjects++;
}
/** Return numberOfObjects */
static int getNumberOfObjects() {
return numberOfObjects;
}
/** Return the area of this circle */
double getArea() {
return radius * radius * Math.PI;
}
}
Method getNumberOfObjects() in CircleWithStaticMembers is a static method. All
the methods in the Math class are static. The main method is static, too.
9.7 Static Variables, Constants, and Methods 339
Instance methods (e.g., getArea()) and instance data (e.g., radius) belong to instances
and can be used only after the instances are created. They are accessed via a reference variable.
Static methods (e.g., getNumberOfObjects()) and static data (e.g., numberOfObjects)
can be accessed from a reference variable or from their class name.
The program in Listing 9.7 demonstrates how to use instance and static variables and methods and illustrates the effects of using them.
LISTING 9.7 TestCircleWithStaticMembers.java
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
public class TestCircleWithStaticMembers {
/** Main method */
public static void main(String[] args) {
System.out.println("Before creating objects");
System.out.println("The number of Circle objects is " +
CircleWithStaticMembers.numberOfObjects);
static variable
// Create c1
CircleWithStaticMembers c1 = new CircleWithStaticMembers();
// Display c1 BEFORE c2 is created
System.out.println("\nAfter creating c1");
System.out.println("c1: radius (" + c1.radius +
") and number of Circle objects (" +
c1.numberOfObjects + ")");
instance variable
static variable
// Create c2
CircleWithStaticMembers c2 = new CircleWithStaticMembers(5);
// Modify c1
c1.radius = 9;
// Display c1 and c2 AFTER c2 was created
System.out.println("\nAfter creating c2 and modifying c1");
System.out.println("c1: radius (" + c1.radius +
") and number of Circle objects (" +
c1.numberOfObjects + ")");
System.out.println("c2: radius (" + c2.radius +
") and number of Circle objects (" +
c2.numberOfObjects + ")");
}
}
Before creating objects
The number of Circle objects is 0
After creating c1
c1: radius (1.0) and number of Circle objects (1)
After creating c2 and modifying c1
c1: radius (9.0) and number of Circle objects (2)
c2: radius (5.0) and number of Circle objects (2)
When you compile TestCircleWithStaticMembers.java, the Java compiler automatically compiles CircleWithStaticMembers.java if it has not been compiled since the last
change.
Static variables and methods can be accessed without creating objects. Line 6 displays the
number of objects, which is 0, since no objects have been created.
instance variable
static variable
static variable
340 Chapter 9
Objects and Classes
The main method creates two circles, c1 and c2 (lines 9, 18). The instance variable
radius in c1 is modified to become 9 (line 21). This change does not affect the instance
variable radius in c2, since these two instance variables are independent. The static variable numberOfObjects becomes 1 after c1 is created (line 9), and it becomes 2 after c2 is
created (line 18).
Note that PI is a constant defined in Math, and Math.PI references the constant. c1.numberOfObjects (line 27) and c2.numberOfObjects (line 30) are better
replaced by CircleWithStaticMembers.numberOfObjects. This improves readability,
because other programmers can easily recognize the static variable. You can also replace
CircleWithStaticMembers.numberOfObjects with CircleWithStaticMembers.
getNumberOfObjects().
Tip
Use ClassName.methodName(arguments) to invoke a static method and
ClassName.staticVariable to access a static variable. This improves readability,
because this makes the static method and data easy to spot.
use class name
An instance method can invoke an instance or static method and access an instance or static
data field. A static method can invoke a static method and access a static data field. However,
a static method cannot invoke an instance method or access an instance data field, since static
methods and static data fields don’t belong to a particular object. The relationship between
static and instance members is summarized in the following diagram:
invoke
access
An instance method
invoke
An instance method
access
An instance data field
A static method
invoke
A static method
access
A static data field
invoke
access
An instance method
An instance data field
A static method
A static data field
For example, the following code is wrong.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
public class A {
int i = 5;
static int k = 2;
public static void main(String[] args) {
int j = i; // Wrong because i is an instance variable
m1(); // Wrong because m1() is an instance method
}
public void m1() {
// Correct since instance and static variables and methods
// can be used in an instance method
i = i + k + m2(i, k);
}
public static int m2(int i, int j) {
return (int)(Math.pow(i, j));
}
}
9.7 Static Variables, Constants, and Methods 341
Note that if you replace the preceding code with the following new code, the program would
be fine, because the instance data field i and method m1 are now accessed from an object a
(lines 7–8):
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
public class A {
int i = 5;
static int k = 2;
public static void main(String[] args) {
A a = new A();
int j = a.i; // OK, a.i accesses the object's instance variable
a.m1(); // OK. a.m1() invokes the object's instance method
}
public void m1() {
i = i + k + m2(i, k);
}
public static int m2(int i, int j) {
return (int)(Math.pow(i, j));
}
}
Design Guide
How do you decide whether a variable or a method should be an instance one or a
static one? A variable or a method that is dependent on a specific instance of the class
should be an instance variable or method. A variable or a method that is not dependent
on a specific instance of the class should be a static variable or method. For example,
every circle has its own radius, so the radius is dependent on a specific circle. Therefore,
radius is an instance variable of the Circle class. Since the getArea method is
dependent on a specific circle, it is an instance method. None of the methods in the
Math class, such as random, pow, sin, and cos, is dependent on a specific instance.
Therefore, these methods are static methods. The main method is static and can be
invoked directly from a class.
instance or static?
Caution
It is a common design error to define an instance method that should have been defined
as static. For example, the method factorial(int n) should be defined as static,
as shown next, because it is independent of any specific instance.
public class Test {
public int factorial(int n) {
int result = 1;
for (int i = 1; i <= n; i ++)
result *= i;
public class Test {
public static int factorial(int n) {
int result = 1;
for (int i = 1; i <= n; i++)
result *= i;
return result;
return result;
}
}
}
}
(a) Wrong design
9.17
common design error
(b) Correct design
Suppose that the class F is defined in (a). Let f be an instance of F. Which of the
statements in (b) are correct?
✓
Check
Point
342 Chapter 9
Objects and Classes
public class F {
int i;
static String s;
void imethod() {
}
static void smethod() {
}
System.out.println(f.i);
System.out.println(f.s);
f.imethod();
f.smethod();
System.out.println(F.i);
System.out.println(F.s);
F.imethod();
F.smethod();
}
(a)
9.18
(b)
Add the static keyword in the place of ? if appropriate.
public class Test {
int count;
public ? void main(String[] args) {
...
}
public ? int getCount() {
return count;
}
public ? int factorial(int n) {
int result = 1;
for (int i = 1; i <= n; i++)
result *= i;
return result;
}
}
9.19
Can you invoke an instance method or reference an instance variable from a static
method? Can you invoke a static method or reference a static variable from an
instance method? What is wrong in the following code?
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
public class C {
public static void main(String[] args) {
method1();
}
public void method1() {
method2();
}
public static void method2() {
System.out.println("What is radius " + c.getRadius());
}
Circle c = new Circle();
}
9.8 Visibility Modifiers
Key
Point
package-private (or
package-access)
Visibility modifiers can be used to specify the visibility of a class and its members.
You can use the public visibility modifier for classes, methods, and data fields to denote that
they can be accessed from any other classes. If no visibility modifier is used, then by default
the classes, methods, and data fields are accessible by any class in the same package. This is
known as package-private or package-access.
9.8 Visibility Modifiers 343
Note
Packages can be used to organize classes. To do so, you need to add the following line
as the first noncomment and nonblank statement in the program:
using packages
package packageName;
If a class is defined without the package statement, it is said to be placed in the default
package.
Java recommends that you place classes into packages rather than using a default package. For simplicity, however, this book uses default packages. For more information on
packages, see Supplement III.E, Packages.
In addition to the public and default visibility modifiers, Java provides the private and
protected modifiers for class members. This section introduces the private modifier.
The protected modifier will be introduced in Section 11.14, The protected Data and
Methods.
The private modifier makes methods and data fields accessible only from within its own
class. Figure 9.14 illustrates how a public, default, and private data field or method in class
C1 can be accessed from a class C2 in the same package and from a class C3 in a different
package.
package p1;
package p1;
package p2;
public class C1 {
public int x;
int y;
private int z;
public class C2 {
void aMethod() {
C1 o = new C1();
can access o.x;
can access o.y;
cannot access o.z;
public class C3 {
void aMethod() {
C1 o = new C1();
can access o.x;
cannot access o.y;
cannot access o.z;
public void m1() {
}
void m2() {
}
private void m3() {
}
}
can invoke o.m1();
can invoke o.m2();
cannot invoke o.m3();
can invoke o.m1();
cannot invoke o.m2();
cannot invoke o.m3();
}
}
}
}
FIGURE 9.14 The private modifier restricts access to its defining class, the default modifier restricts access to a package,
and the public modifier enables unrestricted access.
If a class is not defined as public, it can be accessed only within the same package. As
shown in Figure 9.15, C1 can be accessed from C2 but not from C3.
package p1;
package p1;
package p2;
class C1 {
...
}
public class C2 {
can access C1
}
public class C3 {
cannot access C1;
can access C2;
}
FIGURE 9.15 A nonpublic class has package-access.
A visibility modifier specifies how data fields and methods in a class can be accessed from
outside the class. There is no restriction on accessing data fields and methods from inside the
class. As shown in Figure 9.16b, an object c of class C cannot access its private members,
because c is in the Test class. As shown in Figure 9.16a, an object c of class C can access its
private members, because c is defined inside its own class.
inside access
344 Chapter 9
Objects and Classes
public class C {
private boolean x;
public static void main(String[] args) {
C c = new C();
System.out.println(c.x);
System.out.println(c.convert());
}
public class Test {
public static void main(String[] args) {
C c = new C();
System.out.println(c.x);
System.out.println(c.convert());
}
}
private int convert() {
return x ? 1 : -1;
}
}
(a) This is okay because object c is used inside the class C.
(b) This is wrong because x and convert are private in class C.
FIGURE 9.16 An object can access its private members if it is defined in its own class.
Caution
The private modifier applies only to the members of a class. The public modifier
can apply to a class or members of a class. Using the modifiers public and private
on local variables would cause a compile error.
Note
In most cases, the constructor should be public. However, if you want to prohibit the
user from creating an instance of a class, use a private constructor. For example, there
is no reason to create an instance from the Math class, because all of its data fields and
methods are static. To prevent the user from creating objects from the Math class, the
constructor in java.lang.Math is defined as follows:
private constructor
private Math() {
}
9.9 Data Field Encapsulation
Key
Point
Data field encapsulation
Making data fields private protects data and makes the class easy to maintain.
The data fields radius and numberOfObjects in the CircleWithStaticMembers class in
Listing 9.6 can be modified directly (e.g., c1.radius = 5 or CircleWithStaticMembers
.numberOfObjects = 10). This is not a good practice—for two reasons:
■
First, data may be tampered with. For example, numberOfObjects is to count the
number of objects created, but it may be mistakenly set to an arbitrary value (e.g.,
CircleWithStaticMembers.numberOfObjects = 10).
■
Second, the class becomes difficult to maintain and vulnerable to bugs. Suppose you want to modify the CircleWithStaticMembers class to ensure that
the radius is nonnegative after other programs have already used the class. You
have to change not only the CircleWithStaticMembers class but also the
programs that use it, because the clients may have modified the radius directly
(e.g., c1.radius = -5).
VideoNote
Data field encapsulation
data field encapsulation
To prevent direct modifications of data fields, you should declare the data fields private,
using the private modifier. This is known as data field encapsulation.
9.9 Data Field Encapsulation 345
A private data field cannot be accessed by an object from outside the class that defines
the private field. However, a client often needs to retrieve and modify a data field. To make
a private data field accessible, provide a getter method to return its value. To enable a private
data field to be updated, provide a setter method to set a new value. A getter method is also
referred to as an accessor and a setter to a mutator.
A getter method has the following signature:
getter (or accessor)
setter (or mutator)
public returnType getPropertyName()
If the returnType is boolean, the getter method should be defined as follows by convention:
boolean accessor
public boolean isPropertyName()
A setter method has the following signature:
public void setPropertyName(dataType propertyValue)
Let’s create a new circle class with a private data-field radius and its associated accessor and
mutator methods. The class diagram is shown in Figure 9.17. The new circle class, named
CircleWithPrivateDataFields, is defined in Listing 9.8:
The - sign indicates
a private modifier
Circle
-radius: double
-numberOfObjects: int
The radius of this circle (default: 1.0).
The number of circle objects created.
+Circle()
Constructs a default circle object.
+Circle(radius: double)
Constructs a circle object with the specified radius.
+getRadius(): double
Returns the radius of this circle.
Sets a new radius for this circle.
+setRadius(radius: double): void
+getNumberOfObjects(): int
+getArea(): double
Returns the number of circle objects created.
Returns the area of this circle.
FIGURE 9.17 The Circle class encapsulates circle properties and provides getter/setter and other methods.
LISTING 9.8 CircleWithPrivateDataFields.java
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
public class CircleWithPrivateDataFields {
/** The radius of the circle */
private double radius = 1;
/** The number of objects created */
private static int numberOfObjects = 0;
encapsulate radius
encapsulate
numberOfObjects
/** Construct a circle with radius 1 */
public CircleWithPrivateDataFields() {
numberOfObjects++;
}
/** Construct a circle with a specified radius */
public CircleWithPrivateDataFields(double newRadius) {
radius = newRadius;
numberOfObjects++;
346 Chapter 9
accessor method
mutator method
accessor method
Objects and Classes
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
}
/** Return radius */
public double getRadius() {
return radius;
}
/** Set a new radius */
public void setRadius(double newRadius) {
radius = (newRadius >= 0) ? newRadius : 0;
}
/** Return numberOfObjects */
public static int getNumberOfObjects() {
return numberOfObjects;
}
/** Return the area of this circle */
public double getArea() {
return radius * radius * Math.PI;
}
}
The getRadius() method (lines 20–22) returns the radius, and the setRadius(newRadius)
method (line 25–27) sets a new radius for the object. If the new radius is negative, 0 is set as
the radius for the object. Since these methods are the only ways to read and modify the radius,
you have total control over how the radius property is accessed. If you have to change the
implementation of these methods, you don’t need to change the client programs. This makes
the class easy to maintain.
Listing 9.9 gives a client program that uses the Circle class to create a Circle object and
modifies the radius using the setRadius method.
LISTING 9.9 TestCircleWithPrivateDataFields.java
invoke public method
invoke public method
invoke public method
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
public class TestCircleWithPrivateDataFields {
/** Main method */
public static void main(String[] args) {
// Create a circle with radius 5.0
CircleWithPrivateDataFields myCircle =
new CircleWithPrivateDataFields(5.0);
System.out.println("The area of the circle of radius "
+ myCircle.getRadius() + " is " + myCircle.getArea());
// Increase myCircle's radius by 10%
myCircle.setRadius(myCircle.getRadius() * 1.1);
System.out.println("The area of the circle of radius "
+ myCircle.getRadius() + " is " + myCircle.getArea());
System.out.println("The number of objects created is "
+ CircleWithPrivateDataFields.getNumberOfObjects());
}
}
The data field radius is declared private. Private data can be accessed only within their
defining class, so you cannot use myCircle.radius in the client program. A compile error
would occur if you attempted to access private data from a client.
Since numberOfObjects is private, it cannot be modified. This prevents tampering. For
example, the user cannot set numberOfObjects to 100. The only way to make it 100 is to
create 100 objects of the Circle class.
9.10 Passing Objects to Methods 347
Suppose you combined TestCircleWithPrivateDataFields and Circle into one
class by moving the main method in TestCircleWithPrivateDataFields into Circle.
Could you use myCircle.radius in the main method? See Checkpoint Question 9.22 for
the answer.
Design Guide
To prevent data from being tampered with and to make the class easy to maintain,
declare data fields private.
9.20
9.21
9.22
What is an accessor method? What is a mutator method? What are the naming conventions for accessor methods and mutator methods?
What are the benefits of data field encapsulation?
✓
Check
Point
In the following code, radius is private in the Circle class, and myCircle is an
object of the Circle class. Does the highlighted code cause any problems? If so,
explain why.
public class Circle {
private double radius = 1;
/** Find the area of this circle */
public double getArea() {
return radius * radius * Math.PI;
}
public static void main(String[] args) {
Circle myCircle = new Circle();
System.out.println("Radius is " + myCircle.radius);
}
}
9.10 Passing Objects to Methods
Passing an object to a method is to pass the reference of the object.
You can pass objects to methods. Like passing an array, passing an object is actually passing
the reference of the object. The following code passes the myCircle object as an argument
to the printCircle method:
1
2
3
4
5
6
7
8
9
10
11
12
13
public class Test {
public static void main(String[] args) {
// CircleWithPrivateDataFields is defined in Listing 9.8
CircleWithPrivateDataFields myCircle = new
CircleWithPrivateDataFields(5.0);
printCircle(myCircle);
}
Key
Point
pass an object
public static void printCircle(CircleWithPrivateDataFields c) {
System.out.println("The area of the circle of radius "
+ c.getRadius() + " is " + c.getArea());
}
}
Java uses exactly one mode of passing arguments: pass-by-value. In the preceding code,
the value of myCircle is passed to the printCircle method. This value is a reference to a
Circle object.
The program in Listing 9.10 demonstrates the difference between passing a primitive type
value and passing a reference value.
pass-by-value
348 Chapter 9
Objects and Classes
LISTING 9.10 TestPassObject.java
pass object
object parameter
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
public class TestPassObject {
/** Main method */
public static void main(String[] args) {
// Create a Circle object with radius 1
CircleWithPrivateDataFields myCircle =
new CircleWithPrivateDataFields(1);
// Print areas for radius 1, 2, 3, 4, and 5.
int n = 5;
printAreas(myCircle, n);
// See myCircle.radius and times
System.out.println("\n" + "Radius is " + myCircle.getRadius());
System.out.println("n is " + n);
}
/** Print a table of areas for radius */
public static void printAreas(
CircleWithPrivateDataFields c, int times) {
System.out.println("Radius \t\tArea");
while (times >= 1) {
System.out.println(c.getRadius() + "\t\t" + c.getArea());
c.setRadius(c.getRadius() + 1);
times——;
}
}
}
Radius
Area
1.0
3.141592653589793
2.0
12.566370614359172
3.0
29.274333882308138
4.0
50.26548245743669
5.0
79.53981633974483
Radius is 6.0
n is 5
pass-by-sharing
The CircleWithPrivateDataFields class is defined in Listing 9.8. The program passes a
CircleWithPrivateDataFields object myCircle and an integer value from n to invoke
printAreas(myCircle, n) (line 10), which prints a table of areas for radii 1, 2, 3, 4, 5,
as shown in the sample output.
Figure 9.18 shows the call stack for executing the methods in the program. Note that the
objects are stored in a heap (see Section 7.6).
When passing an argument of a primitive data type, the value of the argument is passed. In
this case, the value of n (5) is passed to times. Inside the printAreas method, the content
of times is changed; this does not affect the content of n.
When passing an argument of a reference type, the reference of the object is passed. In this
case, c contains a reference for the object that is also referenced via myCircle. Therefore,
changing the properties of the object through c inside the printAreas method has the same
effect as doing so outside the method through the variable myCircle. Pass-by-value on references can be best described semantically as pass-by-sharing; that is, the object referenced in
the method is the same as the object being passed.
9.10 Passing Objects to Methods 349
Stack
Pass-by-value (here
the value is 5)
Activation record for the
printArea method
int times: 5
Circle c: reference
Heap
Pass-by-value
(here the value is
the reference for
the object)
Activation record for the
main method
int n: 5
myCircle: reference
A Circle
object
FIGURE 9.18 The value of n is passed to times, and the reference to myCircle is passed
to c in the printAreas method.
9.23
Describe the difference between passing a parameter of a primitive type and passing
a parameter of a reference type. Show the output of the following programs:
public class Test {
public static void main(String[] args) {
Count myCount = new Count();
int times = 0;
public Count(int c) {
count = c;
}
public Count() {
count = 1;
}
System.out.println("count is " + myCount.count);
System.out.println("times is " + times);
}
public static void increment(Count c, int times) {
c.count++;
times++;
}
}
9.24
Show the output of the following program:
public class Test {
public static void main(String[] args) {
Circle circle1 = new Circle(1);
Circle circle2 = new Circle(2);
swap1(circle1, circle2);
System.out.println("After swap1: circle1 = " +
circle1.radius + " circle2 = " + circle2.radius);
swap2(circle1, circle2);
System.out.println("After swap2: circle1 = " +
circle1.radius + " circle2 = " + circle2.radius);
}
public static void swap1(Circle x, Circle y) {
Circle temp = x;
x = y;
y = temp;
}
Check
Point
public class Count {
public int count;
for (int i = 0; i < 100; i++)
increment(myCount, times);
}
✓
350 Chapter 9
Objects and Classes
public static void swap2(Circle x, Circle y) {
double temp = x.radius;
x.radius = y.radius;
y.radius = temp;
}
}
class Circle {
double radius;
Circle(double newRadius) {
radius = newRadius;
}
}
9.25
Show the output of the following code:
public class Test {
public static void main(String[] args) {
int[] a = {1, 2};
swap(a[0], a[1]);
System.out.println("a[0] = " + a[0]
+ " a[1] = " + a[1]);
}
public class Test {
public static void main(String[] args) {
int[] a = {1, 2};
swap(a);
System.out.println("a[0] = " + a[0]
+ " a[1] = " + a[1]);
}
public static void swap(int n1, int n2) {
int temp = n1;
n1 = n2;
n2 = temp;
}
}
public static void swap(int[] a) {
int temp = a[0];
a[0] = a[1];
a[1] = temp;
}
}
(a)
public class Test {
public static void main(String[] args) {
T t = new T();
swap(t);
System.out.println("e1 = " + t.e1
+ " e2 = " + t.e2);
}
public static void swap(T t) {
int temp = t.e1;
t.e1 = t.e2;
t.e2 = temp;
}
}
(b)
public class Test {
public static void main(String[] args) {
T t1 = new T();
T t2 = new T();
System.out.println("t1's i = " +
t1.i + " and j = " + t1.j);
System.out.println("t2's i = " +
t2.i + " and j = " + t2.j);
}
}
class T {
static int i = 0;
int j = 0;
class T {
int e1 = 1;
int e2 = 2;
}
T() {
i++;
j = 1;
}
}
(c)
(d)
9.11 Array of Objects 351
9.26
What is the output of the following programs?
import java.util.Date;
import java.util.Date;
public class Test {
public static void main(String[] args) {
Date date = null;
m1(date);
System.out.println(date);
}
public class Test {
public static void main(String[] args) {
Date date = new Date(1234567);
m1(date);
System.out.println(date.getTime());
}
public static void m1(Date date) {
date = new Date(7654321);
}
public static void m1(Date date) {
date = new Date();
}
}
}
(a)
(b)
import java.util.Date;
import java.util.Date;
public class Test {
public static void main(String[] args) {
Date date = new Date(1234567);
m1(date);
System.out.println(date.getTime());
}
public class Test {
public static void main(String[] args) {
Date date = new Date(1234567);
m1(date);
System.out.println(date.getTime());
}
public static void m1(Date date) {
date.setTime(7654321);
}
}
public static void m1(Date date) {
date = null;
}
}
(c)
(d)
9.11 Array of Objects
An array can hold objects as well as primitive type values.
Chapter 7, Single-Dimensional Arrays, described how to create arrays of primitive type elements. You can also create arrays of objects. For example, the following statement declares
and creates an array of ten Circle objects:
Circle[] circleArray = new Circle[10];
To initialize circleArray, you can use a for loop like this one:
for (int i = 0; i < circleArray.length; i++) {
circleArray[i] = new Circle();
}
An array of objects is actually an array of reference variables. So, invoking circleArray[1].
getArea() involves two levels of referencing, as shown in Figure 9.19. circleArray
references the entire array; circleArray[1] references a Circle object.
Note
When an array of objects is created using the new operator, each element in the array is
a reference variable with a default value of null.
Key
Point
352 Chapter 9
Objects and Classes
circleArray reference
circleArray[0]
Circle object 0
circleArray[1]
…
Circle object 1
circleArray[9]
Circle object 9
FIGURE 9.19 In an array of objects, an element of the array contains a reference to an
object.
Listing 9.11 gives an example that demonstrates how to use an array of objects. The program summarizes the areas of an array of circles. The program creates circleArray, an
array composed of five Circle objects; it then initializes circle radii with random values and
displays the total area of the circles in the array.
LISTING 9.11 TotalArea.java
array of objects
return array of objects
pass array of objects
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
public class TotalArea {
/** Main method */
public static void main(String[] args) {
// Declare circleArray
CircleWithPrivateDataFields[] circleArray;
// Create circleArray
circleArray = createCircleArray();
// Print circleArray and total areas of the circles
printCircleArray(circleArray);
}
/** Create an array of Circle objects */
public static CircleWithPrivateDataFields[] createCircleArray() {
CircleWithPrivateDataFields[] circleArray =
new CircleWithPrivateDataFields[5];
for (int i = 0; i < circleArray.length; i++) {
circleArray[i] =
new CircleWithPrivateDataFields(Math.random() * 100);
}
// Return Circle array
return circleArray;
}
/** Print an array of circles and their total area */
public static void printCircleArray(
CircleWithPrivateDataFields[] circleArray) {
System.out.printf("%-30s%-15s\n", "Radius", "Area");
for (int i = 0; i < circleArray.length; i++) {
System.out.printf("%-30f%-15f\n", circleArray[i].getRadius(),
circleArray[i].getArea());
}
System.out.println("—————————————————————————————————————————-");
// Compute and display the result
System.out.printf("%-30s%-15f\n", "The total area of circles is",
sum(circleArray) );
}
9.12 Immutable Objects and Classes 353
43
44
45
46
47
48
49
50
51
52
53
54
55
/** Add circle areas */
public static double sum(CircleWithPrivateDataFields[] circleArray) {
// Initialize sum
double sum = 0;
pass array of objects
// Add areas to sum
for (int i = 0; i < circleArray.length; i++)
sum += circleArray[i].getArea();
return sum;
}
}
Radius
Area
70.577708
15649.941866
44.152266
6124.291736
24.867853
1942.792644
5.680718
101.380949
36.734246
4239.280350
—————————————————————————————————————————————The total area of circles is 28056.687544
The program invokes createCircleArray() (line 8) to create an array of five circle
objects. Several circle classes were introduced in this chapter. This example uses the CircleWithPrivateDataFields class introduced in Section 9.9, Data Field Encapsulation.
The circle radii are randomly generated using the Math.random() method (line 21).
The createCircleArray method returns an array of CircleWithPrivateDataFields
objects (line 25). The array is passed to the printCircleArray method, which displays the
radius and area of each circle and the total area of the circles.
The sum of the circle areas is computed by invoking the sum method (line 41), which takes
the array of CircleWithPrivateDataFields objects as the argument and returns a double value for the total area.
9.27
What is wrong in the following code?
1
2
3
4
5
6
7
public class Test {
public static void main(String[] args) {
java.util.Date[] dates = new java.util.Date[10];
System.out.println(dates[0]);
System.out.println(dates[0].toString());
}
}
✓
Check
Point
9.12 Immutable Objects and Classes
You can define immutable classes to create immutable objects. The contents of
immutable objects cannot be changed.
Normally, you create an object and allow its contents to be changed later. However, occasionally it is desirable to create an object whose contents cannot be changed once the object
has been created. We call such an object as immutable object and its class as immutable
class. The String class, for example, is immutable. If you deleted the setter method in the
CircleWithPrivateDataFields class in Listing 9.9, the class would be immutable,
because radius is private and cannot be changed without a setter method.
Key
Point
immutable object
immutable class
354 Chapter 9
Objects and Classes
If a class is immutable, then all its data fields must be private and it cannot contain public
setter methods for any data fields. A class with all private data fields and no mutators is not
necessarily immutable. For example, the following Student class has all private data fields
and no setter methods, but it is not an immutable class.
Student class
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
public class Student {
private int id;
private String name;
private java.util.Date dateCreated;
public Student(int ssn, String newName) {
id = ssn;
name = newName;
dateCreated = new java.util.Date();
}
public int getId() {
return id;
}
public String getName() {
return name;
}
public java.util.Date getDateCreated() {
return dateCreated;
}
}
As shown in the following code, the data field dateCreated is returned using the
getDateCreated() method. This is a reference to a Date object. Through this reference,
the content for dateCreated can be changed.
public class Test {
public static void main(String[] args) {
Student student = new Student(111223333, "John");
java.util.Date dateCreated = student.getDateCreated();
dateCreated.setTime(200000); // Now dateCreated field is changed!
}
}
For a class to be immutable, it must meet the following requirements:
■
All data fields must be private.
■
There can’t be any mutator methods for data fields.
■
No accessor methods can return a reference to a data field that is mutable.
Interested readers may refer to Supplement III.U for an extended discussion on immutable objects.
✓
Check
Point
9.28
9.29
9.30
If a class contains only private data fields and no setter methods, is the class immutable?
If all the data fields in a class are private and of primitive types, and the class doesn’t
contain any setter methods, is the class immutable?
Is the following class immutable?
public class A {
private int[] values;
public int[] getValues() {
9.13 The Scope of Variables 355
return values;
}
}
9.13 The Scope of Variables
The scope of instance and static variables is the entire class, regardless of where the
variables are declared.
Section 6.9 discussed local variables and their scope rules. Local variables are declared and
used inside a method locally. This section discusses the scope rules of all the variables in the
context of a class.
Instance and static variables in a class are referred to as the class’s variables or data fields.
A variable defined inside a method is referred to as a local variable. The scope of a class’s
variables is the entire class, regardless of where the variables are declared. A class’s variables
and methods can appear in any order in the class, as shown in Figure 9.20a. The exception is
when a data field is initialized based on a reference to another data field. In such cases, the
other data field must be declared first, as shown in Figure 9.20b. For consistency, this book
declares data fields at the beginning of the class.
public class Circle {
public double findArea() {
return radius * radius * Math.PI;
}
Key
Point
class’s variables
public class F {
private int i ;
private int j = i + 1;
}
private double radius = 1;
}
(a) The variable radius and method findArea() can
be declared in any order.
(b) i has to be declared before j because j’s initial
value is dependent on i.
FIGURE 9.20 Members of a class can be declared in any order, with one exception.
You can declare a class’s variable only once, but you can declare the same variable name
in a method many times in different nonnesting blocks.
If a local variable has the same name as a class’s variable, the local variable takes precedence
and the class’s variable with the same name is hidden. For example, in the following program,
x is defined both as an instance variable and as a local variable in the method.
public class F {
private int x = 0; // Instance variable
private int y = 0;
public F() {
}
public void p() {
int x = 1; // Local variable
System.out.println("x = " + x);
System.out.println("y = " + y);
}
}
What is the output for f.p(), where f is an instance of F? The output for f.p() is 1 for x
and 0 for y. Here is why:
■ x
is declared as a data field with the initial value of 0 in the class, but it is also
declared in the method p() with an initial value of 1. The latter x is referenced in the
System.out.println statement.
■ y
is declared outside the method p(), but y is accessible inside the method.
hidden variables
356 Chapter 9
Objects and Classes
Tip
To avoid confusion and mistakes, do not use the names of instance or static variables as
local variable names, except for method parameters.
✓
Check
Point
9.31
What is the output of the following program?
public class Test {
private static int i = 0;
private static int j = 0;
public static void main(String[] args) {
int i = 2;
int k = 3;
{
int j = 3;
System.out.println("i + j is " + i + j);
}
k = i + j;
System.out.println("k is " + k);
System.out.println("j is " + j);
}
}
9.14 The this Reference
Key
Point
this keyword
The keyword this refers to the object itself. It can also be used inside a constructor to
invoke another constructor of the same class.
The this keyword is the name of a reference that an object can use to refer to itself. You can
use the this keyword to reference the object’s instance members. For example, the following
code in (a) uses this to reference the object’s radius and invokes its getArea() method
explicitly. The this reference is normally omitted, as shown in (b). However, the this
reference is needed to reference hidden data fields or invoke an overloaded constructor.
public class Circle {
private double radius;
public class Circle {
private double radius;
...
...
public double getArea() {
return this.radius * this.radius * Math.PI;
}
public double getArea() {
return radius * radius * Math.PI;
}
Equivalent
public String toString() {
return "radius: " + radius
+ "area: " + getArea() ;
}
public String toString() {
return "radius: " + this.radius
+ "area: " + this.getArea() ;
}
}
}
(a)
9.14.1
hidden data fields
(b)
Using this to Reference Hidden Data Fields
The this keyword can be used to reference a class’s hidden data fields. For example, a datafield name is often used as the parameter name in a setter method for the data field. In this
case, the data field is hidden in the setter method. You need to reference the hidden data-field
name in the method in order to set a new value to it. A hidden static variable can be accessed
9.14 The this Reference 357
simply by using the ClassName.staticVariable reference. A hidden instance variable
can be accessed by using the keyword this, as shown in Figure 9.21a.
public class F {
private int i = 5;
private static double k = 0;
public void setI(int i) {
this.i = i;
}
public static void setK(double k) {
F.k = k;
}
Suppose that f1 and f2 are two objects of F.
Invoking f1.setI(10) is to execute
this.i = 10, where this refers f1
Invoking f2.setI(45) is to execute
this.i = 45, where this refers f2
Invoking F.setK(33) is to execute
F.k = 33. setK is a static method
// Other methods omitted
}
(a)
(b)
FIGURE 9.21 The keyword this refers to the calling object that invokes the method.
The this keyword gives us a way to reference the object that invokes an instance method.
To invoke f1.setI(10), this.i = i is executed, which assigns the value of parameter i to
the data field i of this calling object f1. The keyword this refers to the object that invokes the
instance method setI, as shown in Figure 9.21b. The line F.k = k means that the value in parameter k is assigned to the static data field k of the class, which is shared by all the objects of the class.
9.14.2
Using this to Invoke a Constructor
The this keyword can be used to invoke another constructor of the same class. For example,
you can rewrite the Circle class as follows:
public class Circle {
private double radius;
public Circle(double radius) {
this.radius = radius;
}
The this keyword is used to reference the hidden
data field radius of the object being constructed.
public Circle() {
this(1.0);
}
The this keyword is used to invoke another
constructor.
...
}
The line this(1.0) in the second constructor invokes the first constructor with a double
value argument.
Note
Java requires that the this(arg-list) statement appear first in the constructor
before any other executable statements.
Tip
If a class has multiple constructors, it is better to implement them using this(arg-list)
as much as possible. In general, a constructor with no or fewer arguments can invoke a
constructor with more arguments using this(arg-list). This syntax often simplifies coding and makes the class easier to read and to maintain.
358 Chapter 9
Objects and Classes
✓
Check
Point
9.32
9.33
Describe the role of the this keyword.
What is wrong in the following code?
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
9.34
public class C {
private int p;
public C() {
System.out.println("C's no-arg constructor invoked");
this(0);
}
public C(int p) {
p = p;
}
public void setP(int p) {
p = p;
}
}
What is wrong in the following code?
public class Test {
private int id;
public void m1() {
this.id = 45;
}
public void m2() {
Test.id = 45;
}
}
KEY TERMS
action 322
anonymous object 331
attribute 322
behavior 322
class 322
class’s variable 355
client 325
constructor 322
data field 322
data field encapsulation 344
default constructor 329
dot operator (.) 330
getter (or accessor) 345
instance 322
instance method 331
instance variable 331
instantiation 322
immutable class 353
immutable object 353
no-arg constructor 329
null value 331
object 322
object-oriented programming (OOP) 322
package-private (or package-access) 342
private constructor 344
property 322
public class 325
reference type 330
reference variable 330
setter (or mutator) 345
state 322
static method 337
static variable 337
this keyword 356
Unified Modeling Language (UML) 323
Quiz 359
CHAPTER SUMMARY
1. A class is a template for objects. It defines the properties of objects and provides
constructors for creating objects and methods for manipulating them.
2. A class is also a data type. You can use it to declare object reference variables. An
object reference variable that appears to hold an object actually contains a reference to
that object. Strictly speaking, an object reference variable and an object are different,
but most of the time the distinction can be ignored.
3. An object is an instance of a class. You use the new operator to create an object, and the
dot operator (.) to access members of that object through its reference variable.
4. An instance variable or method belongs to an instance of a class. Its use is associated
with individual instances. A static variable is a variable shared by all instances of the
same class. A static method is a method that can be invoked without using instances.
5. Every instance of a class can access the class’s static variables and methods. For clarity,
however, it is better to invoke static variables and methods using ClassName.variable
and ClassName.method.
6. Visibility modifiers specify how the class, method, and data are accessed. A public
class, method, or data is accessible to all clients. A private method or data is accessible only inside the class.
7. You can provide a getter (accessor) method or a setter (mutator) method to enable
clients to see or modify the data.
8. A getter method has the signature
public returnType getPropertyName().
If the returnType is boolean, the get method should be defined as public
boolean isPropertyName(). A setter method has the signature public void
setPropertyName(dataType propertyValue).
9. All parameters are passed to methods using pass-by-value. For a parameter of a primitive type, the actual value is passed; for a parameter of a reference type, the reference
for the object is passed.
10. A Java array is an object that can contain primitive type values or object type values.
When an array of objects is created, its elements are assigned the default value of null.
11. Once it is created, an immutable object cannot be modified. To prevent users from
modifying an object, you can define immutable classes.
12. The scope of instance and static variables is the entire class, regardless of where the
variables are declared. Instance and static variables can be declared anywhere in the
class. For consistency, they are declared at the beginning of the class in this book.
13. The keyword this can be used to refer to the calling object. It can also be used inside
a constructor to invoke another constructor of the same class.
QUIZ
Answer the quiz for this chapter online at www.cs.armstrong.edu/liang/intro10e/quiz.html.
360 Chapter 9
Objects and Classes
PROGRAMMING EXERCISES
Pedagogical Note
The exercises in Chapters 9–13 help you achieve three objectives:
three objectives
■ Design classes and draw UML class diagrams.
■ Implement classes from the UML.
■ Use classes to develop applications.
Students can download solutions for the UML diagrams for the even-numbered exercises from the Companion Website, and instructors can download all solutions from
the same site.
Sections 9.2–9.5
9.1
(The Rectangle class) Following the example of the Circle class in Section 9.2,
design a class named Rectangle to represent a rectangle. The class contains:
■
■
■
■
■
9.2
Two double data fields named width and height that specify the width and
height of the rectangle. The default values are 1 for both width and height.
A no-arg constructor that creates a default rectangle.
A constructor that creates a rectangle with the specified width and height.
A method named getArea() that returns the area of this rectangle.
A method named getPerimeter() that returns the perimeter.
Draw the UML diagram for the class and then implement the class. Write a test
program that creates two Rectangle objects—one with width 4 and height 40
and the other with width 3.5 and height 35.9. Display the width, height, area,
and perimeter of each rectangle in this order.
(The Stock class) Following the example of the Circle class in Section 9.2,
design a class named Stock that contains:
■
■
■
■
■
■
A string data field named symbol for the stock’s symbol.
A string data field named name for the stock’s name.
A double data field named previousClosingPrice that stores the stock
price for the previous day.
A double data field named currentPrice that stores the stock price for the
current time.
A constructor that creates a stock with the specified symbol and name.
A method named getChangePercent() that returns the percentage changed
from previousClosingPrice to currentPrice.
Draw the UML diagram for the class and then implement the class. Write a test
program that creates a Stock object with the stock symbol ORCL, the name
Oracle Corporation, and the previous closing price of 34.5. Set a new current
price to 34.35 and display the price-change percentage.
Section 9.6
*9.3
*9.4
(Use the Date class) Write a program that creates a Date object, sets its elapsed
time to 10000, 100000, 1000000, 10000000, 100000000, 1000000000,
10000000000, and 100000000000, and displays the date and time using the
toString() method, respectively.
(Use the Random class) Write a program that creates a Random object with seed
1000 and displays the first 50 random integers between 0 and 100 using the
nextInt(100) method.
Programming Exercises 361
*9.5
(Use the GregorianCalendar class) Java API has the GregorianCalendar class
in the java.util package, which you can use to obtain the year, month, and day of a
date. The no-arg constructor constructs an instance for the current date, and the methods get(GregorianCalendar.YEAR), get(GregorianCalendar.MONTH),
and get(GregorianCalendar.DAY_OF_MONTH) return the year, month, and day.
Write a program to perform two tasks:
■
■
Display the current year, month, and day.
The GregorianCalendar class has the setTimeInMillis(long), which
can be used to set a specified elapsed time since January 1, 1970. Set the value
to 1234567898765L and display the year, month, and day.
Sections 9.7–9.9
*9.6
(Stopwatch) Design a class named StopWatch. The class contains:
■
■
■
■
■
9.7
Private data fields startTime and endTime with getter methods.
A no-arg constructor that initializes startTime with the current time.
A method named start() that resets the startTime to the current time.
A method named stop() that sets the endTime to the current time.
A method named getElapsedTime() that returns the elapsed time for the
stopwatch in milliseconds.
Draw the UML diagram for the class and then implement the class. Write a test
program that measures the execution time of sorting 100,000 numbers using
selection sort.
(The Account class) Design a class named Account that contains:
■
■
■
■
■
■
■
■
■
■
■
■
A private int data field named id for the account (default 0).
A private double data field named balance for the account (default 0).
A private double data field named annualInterestRate that stores the current interest rate (default 0). Assume all accounts have the same interest rate.
A private Date data field named dateCreated that stores the date when the
account was created.
A no-arg constructor that creates a default account.
A constructor that creates an account with the specified id and initial balance.
The accessor and mutator methods for id, balance, and annualInterestRate.
The accessor method for dateCreated.
A method named getMonthlyInterestRate() that returns the monthly
interest rate.
A method named getMonthlyInterest() that returns the monthly interest.
A method named withdraw that withdraws a specified amount from the
account.
A method named deposit that deposits a specified amount to the account.
Draw the UML diagram for the class and then implement the class. (Hint: The
method getMonthlyInterest() is to return monthly interest, not the interest rate.
Monthly interest is balance * monthlyInterestRate. monthlyInterestRate
is annualInterestRate / 12. Note that annualInterestRate is a percentage,
e.g., like 4.5%. You need to divide it by 100.)
Write a test program that creates an Account object with an account ID of 1122,
a balance of $20,000, and an annual interest rate of 4.5%. Use the withdraw
method to withdraw $2,500, use the deposit method to deposit $3,000, and print
the balance, the monthly interest, and the date when this account was created.
362 Chapter 9
Objects and Classes
9.8
(The Fan class) Design a class named Fan to represent a fan. The class contains:
■
VideoNote
The Fan class
■
■
■
■
■
■
■
Three constants named SLOW, MEDIUM, and FAST with the values 1, 2, and 3 to
denote the fan speed.
A private int data field named speed that specifies the speed of the fan (the
default is SLOW).
A private boolean data field named on that specifies whether the fan is on (the
default is false).
A private double data field named radius that specifies the radius of the fan
(the default is 5).
A string data field named color that specifies the color of the fan (the default
is blue).
The accessor and mutator methods for all four data fields.
A no-arg constructor that creates a default fan.
A method named toString() that returns a string description for the fan. If
the fan is on, the method returns the fan speed, color, and radius in one combined string. If the fan is not on, the method returns the fan color and radius
along with the string “fan is off” in one combined string.
Draw the UML diagram for the class and then implement the class. Write a test
program that creates two Fan objects. Assign maximum speed, radius 10, color
yellow, and turn it on to the first object. Assign medium speed, radius 5, color
blue, and turn it off to the second object. Display the objects by invoking their
toString method.
**9.9
(Geometry: n-sided regular polygon) In an n-sided regular polygon, all sides
have the same length and all angles have the same degree (i.e., the polygon is
both equilateral and equiangular). Design a class named RegularPolygon that
contains:
■
■
■
■
■
■
■
■
■
■
A private int data field named n that defines the number of sides in the polygon with default value 3.
A private double data field named side that stores the length of the side with
default value 1.
A private double data field named x that defines the x-coordinate of the polygon’s center with default value 0.
A private double data field named y that defines the y-coordinate of the polygon’s center with default value 0.
A no-arg constructor that creates a regular polygon with default values.
A constructor that creates a regular polygon with the specified number of sides
and length of side, centered at (0, 0).
A constructor that creates a regular polygon with the specified number of sides,
length of side, and x- and y-coordinates.
The accessor and mutator methods for all data fields.
The method getPerimeter() that returns the perimeter of the polygon.
The method getArea() that returns the area of the polygon. The formula for
computing the area of a regular polygon is Area =
n * s2
p
4 * tan¢ ≤
n
.
Draw the UML diagram for the class and then implement the class. Write a test
program that creates three RegularPolygon objects, created using the no-arg
constructor, using RegularPolygon(6, 4), and using RegularPolygon(10,
4, 5.6, 7.8). For each object, display its perimeter and area.
Programming Exercises 363
*9.10
(Algebra: quadratic equations) Design a class named QuadraticEquation for
a quadratic equation ax2 + bx + x = 0. The class contains:
■
■
■
■
■
Private data fields a, b, and c that represent three coefficients.
A constructor for the arguments for a, b, and c.
Three getter methods for a, b, and c.
A method named getDiscriminant() that returns the discriminant, which is
b2 - 4ac.
The methods named getRoot1() and getRoot2() for returning two roots of
the equation
r1 =
*9.11
-b + 2b2 - 4ac
2a
and r2 =
-b - 2b2 - 4ac
2a
These methods are useful only if the discriminant is nonnegative. Let these methods return 0 if the discriminant is negative.
Draw the UML diagram for the class and then implement the class. Write a test
program that prompts the user to enter values for a, b, and c and displays the result
based on the discriminant. If the discriminant is positive, display the two roots. If
the discriminant is 0, display the one root. Otherwise, display “The equation has
no roots.” See Programming Exercise 3.1 for sample runs.
(Algebra: 2 * 2 linear equations) Design a class named LinearEquation for a
2 * 2 system of linear equations:
ed - bf
ax + by = e
x =
ad - bc
cx + dy = f
y =
af - ec
ad - bc
The class contains:
■
■
■
■
■
**9.12
**9.13
Private data fields a, b, c, d, e, and f.
A constructor with the arguments for a, b, c, d, e, and f.
Six getter methods for a, b, c, d, e, and f.
A method named isSolvable() that returns true if ad - bc is not 0.
Methods getX() and getY() that return the solution for the equation.
Draw the UML diagram for the class and then implement the class. Write a test
program that prompts the user to enter a, b, c, d, e, and f and displays the result.
If ad - bc is 0, report that “The equation has no solution.” See Programming
Exercise 3.3 for sample runs.
(Geometry: intersecting point) Suppose two line segments intersect. The two endpoints for the first line segment are (x1, y1) and (x2, y2) and for the second line
segment are (x3, y3) and (x4, y4). Write a program that prompts the user to enter
these four endpoints and displays the intersecting point. As discussed in Programming Exercise 3.25, the intersecting point can be found by solving a linear equation. Use the LinearEquation class in Programming Exercise 9.11 to solve this
equation. See Programming Exercise 3.25 for sample runs.
(The Location class) Design a class named Location for locating a maximal
value and its location in a two-dimensional array. The class contains public data
fields row, column, and maxValue that store the maximal value and its indices
in a two-dimensional array with row and column as int types and maxValue as
a double type.
Write the following method that returns the location of the largest element in a
two-dimensional array:
public static Location locateLargest(double[][] a)
364 Chapter 9
Objects and Classes
The return value is an instance of Location. Write a test program that prompts
the user to enter a two-dimensional array and displays the location of the largest
element in the array. Here is a sample run:
Enter the number of rows and columns in the array:
Enter the array:
23.5 35 2 10
4.5 3 45 3.5
35 44 5.5 9.6
The location of the largest element is 45 at (1, 2)
3 4
CHAPTER
10
OBJECT-ORIENTED
THINKING
Objectives
■
To apply class abstraction to develop software (§10.2).
■
To explore the differences between the procedural paradigm and
object-oriented paradigm (§10.3).
■
To discover the relationships between classes (§10.4).
■
To design programs using the object-oriented paradigm (§§10.5–10.6).
■
To create objects for primitive values using the wrapper classes (Byte,
Short, Integer, Long, Float, Double, Character, and Boolean)
(§10.7).
■
To simplify programming using automatic conversion between
primitive types and wrapper class types (§10.8).
■
To use the BigInteger and BigDecimal classes for computing very
large numbers with arbitrary precisions (§10.9).
■
To use the String class to process immutable strings (§10.10).
■
To use the StringBuilder and StringBuffer classes to process
mutable strings (§10.11).
366 Chapter 10
Object-Oriented Thinking
10.1 Introduction
Key
Point
The focus of this chapter is on class design and explores the differences between
procedural programming and object-oriented programming.
The preceding chapter introduced objects and classes. You learned how to define classes,
create objects, and use objects from several classes in the Java API (e.g., Circle, Date,
Random, and Point2D). This book’s approach is to teach problem solving and fundamental
programming techniques before object-oriented programming. This chapter shows how procedural and object-oriented programming differ. You will see the benefits of object-oriented
programming and learn to use it effectively.
Our focus here is on class design. We will use several examples to illustrate the advantages
of the object-oriented approach. The examples involve designing new classes and using them
in applications and introducing new classes in the Java API.
10.2 Class Abstraction and Encapsulation
Key
Point
class abstraction
class’s contract
class encapsulation
abstract data type
Class abstraction is the separation of class implementation from the use of a class.
The details of implementation are encapsulated and hidden from the user. This is
known as class encapsulation.
In Chapter 6, you learned about method abstraction and used it in stepwise refinement. Java
provides many levels of abstraction, and class abstraction separates class implementation
from how the class is used. The creator of a class describes the functions of the class and lets
the user know how the class can be used. The collection of methods and fields that are accessible from outside the class, together with the description of how these members are expected to
behave, serves as the class’s contract. As shown in Figure 10.1, the user of the class does not
need to know how the class is implemented. The details of implementation are encapsulated
and hidden from the user. This is called class encapsulation. For example, you can create a
Circle object and find the area of the circle without knowing how the area is computed. For
this reason, a class is also known as an abstract data type (ADT).
Class implementation
is like a black box
hidden from the clients
Class
Class Contract
(signatures of
public methods and
public constants)
Clients use the
class through the
contract of the class
FIGURE 10.1 Class abstraction separates class implementation from the use of the class.
Class abstraction and encapsulation are two sides of the same coin. Many real-life examples illustrate the concept of class abstraction. Consider, for instance, building a computer
system. Your personal computer has many components—a CPU, memory, disk, motherboard,
fan, and so on. Each component can be viewed as an object that has properties and methods.
To get the components to work together, you need know only how each component is used
and how it interacts with the others. You don’t need to know how the components work
internally. The internal implementation is encapsulated and hidden from you. You can build a
computer without knowing how a component is implemented.
The computer-system analogy precisely mirrors the object-oriented approach. Each component can be viewed as an object of the class for the component. For example, you might
have a class that models all kinds of fans for use in a computer, with properties such as fan
size and speed and methods such as start and stop. A specific fan is an instance of this class
with specific property values.
As another example, consider getting a loan. A specific loan can be viewed as an object
of a Loan class. The interest rate, loan amount, and loan period are its data properties, and
10.2 Class Abstraction and Encapsulation 367
computing the monthly payment and total payment are its methods. When you buy a car, a
loan object is created by instantiating the class with your loan interest rate, loan amount, and
loan period. You can then use the methods to find the monthly payment and total payment
of your loan. As a user of the Loan class, you don’t need to know how these methods are
implemented.
Listing 2.9, ComputeLoan.java, presented a program for computing loan payments. That
program cannot be reused in other programs because the code for computing the payments is
in the main method. One way to fix this problem is to define static methods for computing
the monthly payment and total payment. However, this solution has limitations. Suppose you
wish to associate a date with the loan. There is no good way to tie a date with a loan without
using objects. The traditional procedural programming paradigm is action-driven, and data are
separated from actions. The object-oriented programming paradigm focuses on objects, and
actions are defined along with the data in objects. To tie a date with a loan, you can define a
loan class with a date along with the loan’s other properties as data fields. A loan object now
contains data and actions for manipulating and processing data, and the loan data and actions
are integrated in one object. Figure 10.2 shows the UML class diagram for the Loan class.
VideoNote
The Loan class
Loan
-annualInterestRate: double
The annual interest rate of the loan (default: 2.5).
-numberOfYears: int
The number of years for the loan (default: 1).
-loanAmount: double
The loan amount (default: 1000).
-loanDate: java.util.Date
The date this loan was created.
+Loan()
Constructs a default Loan object.
+Loan(annualInterestRate: double,
numberOfYears: int,loanAmount:
double)
Constructs a loan with specified interest rate, years,
and loan amount.
+getAnnualInterestRate(): double
Returns the annual interest rate of this loan.
+getNumberOfYears(): int
Returns the number of the years of this loan.
+getLoanAmount(): double
+getLoanDate(): java.util.Date
Returns the amount of this loan.
+setAnnualInterestRate(
annualInterestRate: double): void
Sets a new annual interest rate for this loan.
+setNumberOfYears(
numberOfYears: int): void
+setLoanAmount(
loanAmount: double): void
+getMonthlyPayment(): double
Sets a new number of years for this loan.
+getTotalPayment(): double
Returns the total payment for this loan.
Returns the date of the creation of this loan.
Sets a new amount for this loan.
Returns the monthly payment for this loan.
FIGURE 10.2 The Loan class models the properties and behaviors of loans.
The UML diagram in Figure 10.2 serves as the contract for the Loan class. Throughout this
book, you will play the roles of both class user and class developer. Remember that a class
user can use the class without knowing how the class is implemented.
Assume that the Loan class is available. The program in Listing 10.1 uses that class.
LISTING 10.1 TestLoanClass.java
1
2
3
4
5
import java.util.Scanner;
public class TestLoanClass {
/** Main method */
public static void main(String[] args) {
368 Chapter 10
create Loan object
invoke instance method
invoke instance method
Object-Oriented Thinking
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
// Create a Scanner
Scanner input = new Scanner(System.in);
// Enter annual interest rate
System.out.print(
"Enter annual interest rate, for example, 8.25: ");
double annualInterestRate = input.nextDouble();
// Enter number of years
System.out.print("Enter number of years as an integer: ");
int numberOfYears = input.nextInt();
// Enter loan amount
System.out.print("Enter loan amount, for example, 120000.95: ");
double loanAmount = input.nextDouble();
// Create a Loan object
Loan loan =
new Loan(annualInterestRate, numberOfYears, loanAmount);
// Display loan date, monthly payment, and total payment
System.out.printf("The loan was created on %s\n" +
"The monthly payment is %.2f\nThe total payment is %.2f\n",
loan.getLoanDate().toString(), loan.getMonthlyPayment(),
loan.getTotalPayment());
}
}
Enter annual interest rate, for example, 8.25: 2.5
Enter number of years as an integer: 5
Enter loan amount, for example, 120000.95: 1000
The loan was created on Sat Jun 16 21:12:50 EDT 2012
The monthly payment is 17.75
The total payment is 1064.84
The main method reads the interest rate, the payment period (in years), and the loan amount;
creates a Loan object; and then obtains the monthly payment (line 29) and the total payment
(line 30) using the instance methods in the Loan class.
The Loan class can be implemented as in Listing 10.2.
LISTING 10.2 Loan.java
no-arg constructor
1
2
3
4
5
6
7
8
9
10
11
12
public class Loan {
private double annualInterestRate;
private int numberOfYears;
private double loanAmount;
private java.util.Date loanDate;
/** Default constructor */
public Loan() {
this(2.5, 1, 1000);
}
/** Construct a loan with specified annual interest rate,
10.2 Class Abstraction and Encapsulation 369
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
number of years, and loan amount
*/
public Loan(double annualInterestRate, int numberOfYears,
double loanAmount) {
this.annualInterestRate = annualInterestRate;
this.numberOfYears = numberOfYears;
this.loanAmount = loanAmount;
loanDate = new java.util.Date();
}
/** Return annualInterestRate */
public double getAnnualInterestRate() {
return annualInterestRate;
}
/** Set a new annualInterestRate */
public void setAnnualInterestRate(double annualInterestRate) {
this.annualInterestRate = annualInterestRate;
}
/** Return numberOfYears */
public int getNumberOfYears() {
return numberOfYears;
}
/** Set a new numberOfYears */
public void setNumberOfYears(int numberOfYears) {
this.numberOfYears = numberOfYears;
}
/** Return loanAmount */
public double getLoanAmount() {
return loanAmount;
}
/** Set a new loanAmount */
public void setLoanAmount(double loanAmount) {
this.loanAmount = loanAmount;
}
/** Find monthly payment */
public double getMonthlyPayment() {
double monthlyInterestRate = annualInterestRate / 1200;
double monthlyPayment = loanAmount * monthlyInterestRate / (1 (1 / Math.pow(1 + monthlyInterestRate, numberOfYears * 12)));
return monthlyPayment;
}
/** Find total payment */
public double getTotalPayment() {
double totalPayment = getMonthlyPayment() * numberOfYears * 12;
return totalPayment;
}
/** Return loan date */
public java.util.Date getLoanDate() {
return loanDate;
}
}
constructor
370 Chapter 10
Object-Oriented Thinking
From a class developer’s perspective, a class is designed for use by many different customers.
In order to be useful in a wide range of applications, a class should provide a variety of ways
for customization through constructors, properties, and methods.
The Loan class contains two constructors, four getter methods, three setter methods,
and the methods for finding the monthly payment and the total payment. You can construct a Loan object by using the no-arg constructor or the constructor with three parameters: annual interest rate, number of years, and loan amount. When a loan object is created,
its date is stored in the loanDate field. The getLoanDate method returns the date. The
methods—getAnnualInterest, getNumberOfYears, and getLoanAmount—return the
annual interest rate, payment years, and loan amount, respectively. All the data properties
and methods in this class are tied to a specific instance of the Loan class. Therefore, they are
instance variables and methods.
Important Pedagogical Tip
Use the UML diagram for the Loan class shown in Figure 10.2 to write a test program
that uses the Loan class even though you don’t know how the Loan class is implemented. This has three benefits:
■ It demonstrates that developing a class and using a class are two separate tasks.
■ It enables you to skip the complex implementation of certain classes without inter-
rupting the sequence of this book.
■ It is easier to learn how to implement a class if you are familiar with it by using the class.
For all the class examples from now on, create an object from the class and try using its
methods before turning your attention to its implementation.
✓
Check
Point
10.1
If you redefine the Loan class in Listing 10.2 without setter methods, is the class
immutable?
10.3 Thinking in Objects
Key
Point
The procedural paradigm focuses on designing methods. The object-oriented
paradigm couples data and methods together into objects. Software design using the
object-oriented paradigm focuses on objects and operations on objects.
Chapters 1–8 introduced fundamental programming techniques for problem solving using
loops, methods, and arrays. Knowing these techniques lays a solid foundation for objectoriented programming. Classes provide more flexibility and modularity for building reusable
software. This section improves the solution for a problem introduced in Chapter 3 using the
object-oriented approach. From these improvements, you will gain insight into the differences
between procedural and object-oriented programming and see the benefits of developing reusable code using objects and classes.
Listing 3.4, ComputeAndInterpretBMI.java, presented a program for computing body
mass index. The code cannot be reused in other programs, because the code is in the main
method. To make it reusable, define a static method to compute body mass index as follows:
public static double getBMI(double weight, double height)
VideoNote
The BMI class
This method is useful for computing body mass index for a specified weight and height. However, it has limitations. Suppose you need to associate the weight and height with a person’s
name and birth date. You could declare separate variables to store these values, but these
values would not be tightly coupled. The ideal way to couple them is to create an object that
contains them all. Since these values are tied to individual objects, they should be stored in
instance data fields. You can define a class named BMI as shown in Figure 10.3.
10.3 Thinking in Objects 371
The getter methods for these data fields
are provided in the class, but omitted in the
UML diagram for brevity.
BMI
The name of the person.
-name: String
-age: int
-weight: double
-height: double
The age of the person.
The weight of the person in pounds.
The height of the person in inches.
+BMI(name: String, age: int, weight:
double, height: double)
Creates a BMI object with the specified
name, age, weight, and height.
+BMI(name: String, weight: double,
height: double)
Creates a BMI object with the specified
name, weight, height, and a default age 20.
+getBMI(): double
+getStatus(): String
Returns the BMI.
Returns the BMI status (e.g., normal,
overweight, etc.).
FIGURE 10.3 The BMI class encapsulates BMI information.
Assume that the BMI class is available. Listing 10.3 gives a test program that uses this class.
LISTING 10.3 UseBMIClass.java
1
2
3
4
5
6
7
8
9
10
11
public class UseBMIClass {
public static void main(String[] args) {
BMI bmi1 = new BMI("Kim Yang", 18, 145, 70);
System.out.println("The BMI for " + bmi1.getName() + " is "
+ bmi1.getBMI() + " " + bmi1.getStatus());
BMI bmi2 = new BMI("Susan King", 215, 70);
System.out.println("The BMI for " + bmi2.getName() + " is "
+ bmi2.getBMI() + " " + bmi2.getStatus());
create an object
invoke instance method
create an object
invoke instance method
}
}
The BMI for Kim Yang is 20.81 Normal
The BMI for Susan King is 30.85 Obese
Line 3 creates the object bmi1 for Kim Yang and line 7 creates the object bmi2 for Susan
King. You can use the instance methods getName(), getBMI(), and getStatus() to
return the BMI information in a BMI object.
The BMI class can be implemented as in Listing 10.4.
LISTING 10.4 BMI.java
1
2
3
4
5
6
7
8
9
10
public class BMI {
private String name;
private int age;
private double weight; // in pounds
private double height; // in inches
public static final double KILOGRAMS_PER_POUND = 0.45359237;
public static final double METERS_PER_INCH = 0.0254;
public BMI(String name, int age, double weight, double height) {
this.name = name;
constructor
372 Chapter 10
Object-Oriented Thinking
constructor
getBMI
getStatus
procedural vs. object-oriented
paradigms
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
this.age = age;
this.weight = weight;
this.height = height;
}
public BMI(String name, double weight, double height) {
this(name, 20, weight, height);
}
public double getBMI() {
double bmi = weight * KILOGRAMS_PER_POUND /
((height * METERS_PER_INCH) * (height * METERS_PER_INCH));
return Math.round(bmi * 100) / 100.0;
}
public String getStatus() {
double bmi = getBMI();
if (bmi < 18.5)
return "Underweight";
else if (bmi < 25)
return "Normal";
else if (bmi < 30)
return "Overweight";
else
return "Obese";
}
public String getName() {
return name;
}
public int getAge() {
return age;
}
public double getWeight() {
return weight;
}
public double getHeight() {
return height;
}
}
The mathematical formula for computing the BMI using weight and height is given in
Section 3.8. The instance method getBMI() returns the BMI. Since the weight and height are
instance data fields in the object, the getBMI() method can use these properties to compute
the BMI for the object.
The instance method getStatus() returns a string that interprets the BMI. The interpretation is also given in Section 3.8.
This example demonstrates the advantages of the object-oriented paradigm over the procedural paradigm. The procedural paradigm focuses on designing methods. The object-oriented
paradigm couples data and methods together into objects. Software design using the objectoriented paradigm focuses on objects and operations on objects. The object-oriented approach
combines the power of the procedural paradigm with an added dimension that integrates data
with operations into objects.
In procedural programming, data and operations on the data are separate, and this methodology requires passing data to methods. Object-oriented programming places data and
10.4 Class Relationships 373
the operations that pertain to them in an object. This approach solves many of the problems
inherent in procedural programming. The object-oriented programming approach organizes
programs in a way that mirrors the real world, in which all objects are associated with both
attributes and activities. Using objects improves software reusability and makes programs
easier to develop and easier to maintain. Programming in Java involves thinking in terms of
objects; a Java program can be viewed as a collection of cooperating objects.
10.2
✓
Is the BMI class defined in Listing 10.4 immutable?
Check
Point
10.4 Class Relationships
To design classes, you need to explore the relationships among classes. The common
relationships among classes are association, aggregation, composition, and
inheritance.
Key
Point
This section explores association, aggregation, and composition. The inheritance relationship
will be introduced in the next chapter.
10.4.1
Association
Association is a general binary relationship that describes an activity between two classes.
For example, a student taking a course is an association between the Student class and the
Course class, and a faculty member teaching a course is an association between the Faculty
class and the Course class. These associations can be represented in UML graphical notation,
as shown in Figure 10.4.
Take
Student
5..60
association
Teach
Course
0..3
1
Teacher
Faculty
FIGURE 10.4 This UML diagram shows that a student may take any number of courses, a
faculty member may teach at most three courses, a course may have from five to sixty students, and a course is taught by only one faculty member.
An association is illustrated by a solid line between two classes with an optional label that
describes the relationship. In Figure 10.4, the labels are Take and Teach. Each relationship
may have an optional small black triangle that indicates the direction of the relationship. In
this figure, the direction indicates that a student takes a course (as opposed to a course taking
a student).
Each class involved in the relationship may have a role name that describes the role it plays
in the relationship. In Figure 10.4, teacher is the role name for Faculty.
Each class involved in an association may specify a multiplicity, which is placed at the
side of the class to specify how many of the class’s objects are involved in the relationship
in UML. A multiplicity could be a number or an interval that specifies how many of the
class’s objects are involved in the relationship. The character * means an unlimited number
of objects, and the interval m..n indicates that the number of objects is between m and n,
inclusively. In Figure 10.4, each student may take any number of courses, and each course
must have at least five and at most sixty students. Each course is taught by only one faculty
member, and a faculty member may teach from zero to three courses per semester.
In Java code, you can implement associations by using data fields and methods. For example, the relationships in Figure 10.4 may be implemented using the classes in Figure 10.5. The
multiplicity
374 Chapter 10
Object-Oriented Thinking
relation “a student takes a course” is implemented using the addCourse method in the Student class and the addStuent method in the Course class. The relation “a faculty teaches
a course” is implemented using the addCourse method in the Faculty class and the setFaculty method in the Course class. The Student class may use a list to store the courses
that the student is taking, the Faculty class may use a list to store the courses that the faculty
is teaching, and the Course class may use a list to store students enrolled in the course and a
data field to store the instructor who teaches the course.
public class Student {
private Course[]
courseList;
public class Course {
private Student[]
classList;
private Faculty faculty;
public void addCourse(
Course s) { ... }
public class Faculty {
private Course[]
courseList;
public void addCourse(
Course c) { ... }
public void addStudent(
Student s) { ... }
}
}
public void setFaculty(
Faculty faculty) { ... }
}
FIGURE 10.5 The association relations are implemented using data fields and methods in classes.
Note
There are many possible ways to implement relationships. For example, the student
and faculty information in the Course class can be omitted, since they are already in
the Student and Faculty class. Likewise, if you don’t need to know the courses a
student takes or a faculty member teaches, the data field courseList and the addCourse method in Student or Faculty can be omitted.
many possible
implementations
10.4.2
aggregation
aggregating object
aggregated object
aggregated class
aggregating class
composition
Aggregation and Composition
Aggregation is a special form of association that represents an ownership relationship between
two objects. Aggregation models has-a relationships. The owner object is called an aggregating
object, and its class is called an aggregating class. The subject object is called an aggregated
object, and its class is called an aggregated class.
An object can be owned by several other aggregating objects. If an object is exclusively
owned by an aggregating object, the relationship between the object and its aggregating object
is referred to as a composition. For example, “a student has a name” is a composition relationship between the Student class and the Name class, whereas “a student has an address” is an
aggregation relationship between the Student class and the Address class, since an address
can be shared by several students. In UML, a filled diamond is attached to an aggregating
class (in this case, Student) to denote the composition relationship with an aggregated class
(Name), and an empty diamond is attached to an aggregating class (Student) to denote the
aggregation relationship with an aggregated class (Address), as shown in Figure 10.6.
Composition
Name
1
1
Student
Aggregation
1..3
1
Address
FIGURE 10.6 Each student has a name and an address.
In Figure 10.6, each student has only one multiplicity—address—and each address can
be shared by up to 3 students. Each student has one name, and a name is unique for each
student.
10.4 Class Relationships 375
An aggregation relationship is usually represented as a data field in the aggregating class.
For example, the relationships in Figure 10.6 may be implemented using the classes in
Figure 10.7. The relation “a student has a name” and “a student has an address” are implemented in the data field name and address in the Student class.
public class Name {
...
}
public class Student {
private Name name;
private Address address;
...
}
Aggregated class
Aggregating class
public class Address {
...
}
Aggregated class
FIGURE 10.7 The composition relations are implemented using data fields in classes.
Aggregation may exist between objects of the same class. For example, a person may have a
supervisor. This is illustrated in Figure 10.8.
Person
1
1
Supervisor
FIGURE 10.8 A person may have a supervisor.
In the relationship “a person has a supervisor,” a supervisor can be represented as a data
field in the Person class, as follows:
public class Person {
// The type for the data is the class itself
private Person supervisor;
...
}
If a person can have several supervisors, as shown in Figure 10.9a, you may use an array to
store supervisors, as shown in Figure 10.9b.
1
Person
Supervisor
m
public class Person {
...
private Person[] supervisors;
}
(a)
(b)
FIGURE 10.9 A person can have several supervisors.
Note
Since aggregation and composition relationships are represented using classes in the
same way, we will not differentiate them and call both compositions for simplicity.
10.3
10.4
10.5
10.6
What are common relationships among classes?
What is association? What is aggregation? What is composition?
What is UML notation of aggregation and composition?
Why both aggregation and composition are together referred to as composition?
aggregation or composition
✓
Check
Point
376 Chapter 10
Object-Oriented Thinking
10.5 Case Study: Designing the Course Class
Key
Point
This section designs a class for modeling courses.
This book’s philosophy is teaching by example and learning by doing. The book provides a
wide variety of examples to demonstrate object-oriented programming. This section and the
next offer additional examples on designing classes.
Suppose you need to process course information. Each course has a name and has students
enrolled. You should be able to add/drop a student to/from the course. You can use a class to
model the courses, as shown in Figure 10.10.
Course
-courseName: String
-students: String[]
-numberOfStudents: int
The name of the course.
An array to store the students for the course.
The number of students (default: 0).
+Course(courseName: String)
+getCourseName(): String
Creates a course with the specified name.
Returns the course name.
+addStudent(student: String): void
Adds a new student to the course.
+dropStudent(student: String): void
+getStudents(): String[]
+getNumberOfStudents(): int
Drops a student from the course.
Returns the students for the course.
Returns the number of students for the course.
FIGURE 10.10 The Course class models the courses.
A Course object can be created using the constructor Course(String name) by passing
a course name. You can add students to the course using the addStudent(String student)
method, drop a student from the course using the dropStudent(String student) method,
and return all the students in the course using the getStudents() method. Suppose the Course
class is available; Listing 10.5 gives a test class that creates two courses and adds students to them.
LISTING 10.5 TestCourse.java
create a course
add a student
number of students
return students
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
public class TestCourse {
public static void main(String[] args) {
Course course1 = new Course("Data Structures");
Course course2 = new Course("Database Systems");
course1.addStudent("Peter Jones");
course1.addStudent("Kim Smith");
course1.addStudent("Anne Kennedy");
course2.addStudent("Peter Jones");
course2.addStudent("Steve Smith");
System.out.println("Number of students in course1: "
+ course1.getNumberOfStudents());
String[] students = course1.getStudents();
for (int i = 0; i < course1.getNumberOfStudents(); i++)
System.out.print(students[i] + ", ");
System.out.println();
System.out.print("Number of students in course2: "
+ course2.getNumberOfStudents());
}
}
10.5 Case Study: Designing the Course Class 377
Number of students in course1: 3
Peter Jones, Kim Smith, Anne Kennedy,
Number of students in course2: 2
The Course class is implemented in Listing 10.6. It uses an array to store the students in the
course. For simplicity, assume that the maximum course enrollment is 100. The array is created using new String[100] in line 3. The addStudent method (line 10) adds a student to
the array. Whenever a new student is added to the course, numberOfStudents is increased
(line 12). The getStudents method returns the array. The dropStudent method (line 27)
is left as an exercise.
LISTING 10.6 Course.java
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
public class Course {
private String courseName;
private String[] students = new String[100];
private int numberOfStudents;
public Course(String courseName) {
this.courseName = courseName;
}
create students
add a course
public void addStudent(String student) {
students[numberOfStudents] = student;
numberOfStudents++;
}
public String[] getStudents() {
return students;
}
return students
public int getNumberOfStudents() {
return numberOfStudents;
}
number of students
public String getCourseName() {
return courseName;
}
public void dropStudent(String student) {
// Left as an exercise in Programming Exercise 10.9
}
}
The array size is fixed to be 100 (line 3), so you cannot have more than 100 students in the
course. You can improve the class by automatically increasing the array size in Programming
Exercise 10.9.
When you create a Course object, an array object is created. A Course object contains a
reference to the array. For simplicity, you can say that the Course object contains the array.
The user can create a Course object and manipulate it through the public methods
addStudent, dropStudent, getNumberOfStudents, and getStudents. However, the
user doesn’t need to know how these methods are implemented. The Course class encapsulates the internal implementation. This example uses an array to store students, but you could
use a different data structure to store students. The program that uses Course does not need
to change as long as the contract of the public methods remains unchanged.
378 Chapter 10
Object-Oriented Thinking
10.6 Case Study: Designing a Class for Stacks
stack
Key
Point
This section designs a class for modeling stacks.
Recall that a stack is a data structure that holds data in a last-in, first-out fashion, as shown in
Figure 10.11.
Data3
Data2
Data1
Data2
Data1
Data1
Data3
Data1
Data2
Data2
Data1
Data3
Data2
Data1
Data1
FIGURE 10.11 A stack holds data in a last-in, first-out fashion.
VideoNote
The StackOfIntegers class
Stacks have many applications. For example, the compiler uses a stack to process method
invocations. When a method is invoked, its parameters and local variables are pushed into a
stack. When a method calls another method, the new method’s parameters and local variables
are pushed into the stack. When a method finishes its work and returns to its caller, its associated space is released from the stack.
You can define a class to model stacks. For simplicity, assume the stack holds the int
values. So name the stack class StackOfIntegers. The UML diagram for the class is shown
in Figure 10.12.
StackOfIntegers
-elements: int[]
-size: int
+StackOfIntegers()
+StackOfIntegers(capacity: int)
+empty(): boolean
+peek(): int
An array to store integers in the stack.
The number of integers in the stack.
+push(value: int): void
Constructs an empty stack with a default capacity of 16.
Constructs an empty stack with a specified capacity.
Returns true if the stack is empty.
Returns the integer at the top of the stack without
removing it from the stack.
Stores an integer into the top of the stack.
+pop(): int
Removes the integer at the top of the stack and returns it.
+getSize(): int
Returns the number of elements in the stack.
FIGURE 10.12 The StackOfIntegers class encapsulates the stack storage and provides
the operations for manipulating the stack.
Suppose that the class is available. The test program in Listing 10.7 uses the class to create a stack (line 3), store ten integers 0, 1, 2, . . . , and 9 (line 6), and displays them in reverse
order (line 9).
LISTING 10.7 TestStackOfIntegers.java
create a stack
1
2
3
public class TestStackOfIntegers {
public static void main(String[] args) {
StackOfIntegers stack = new StackOfIntegers();
10.6 Case Study: Designing a Class for Stacks 379
4
5
6
7
8
9
10
11
for (int i = 0; i < 10; i++)
stack.push(i);
push to stack
while (!stack.empty())
System.out.print(stack.pop() + " ");
pop from stack
}
}
9 8 7 6 5 4 3 2 1 0
How do you implement the StackOfIntegers class? The elements in the stack are stored
in an array named elements. When you create a stack, the array is also created. The no-arg
constructor creates an array with the default capacity of 16. The variable size counts the
number of elements in the stack, and size – 1 is the index of the element at the top of the
stack, as shown in Figure 10.13. For an empty stack, size is 0.
elements[capacity 1]
.
.
.
elements[size 1]
top
capacity
.
.
size
.
elements[1]
elements[0]
bottom
FIGURE 10.13 The StackOfIntegers class encapsulates the stack storage and provides
the operations for manipulating the stack.
The StackOfIntegers class is implemented in Listing 10.8. The methods empty(),
peek(), pop(), and getSize() are easy to implement. To implement push(int value),
assign value to elements[size] if size < capacity (line 24). If the stack is full (i.e.,
size >= capacity), create a new array of twice the current capacity (line 19), copy the contents of the current array to the new array (line 20), and assign the reference of the new array
to the current array in the stack (line 21). Now you can add the new value to the array (line 24).
LISTING 10.8 StackOfIntegers.java
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
public class StackOfIntegers {
private int[] elements;
private int size;
public static final int DEFAULT_CAPACITY = 16;
/** Construct a stack with the default capacity 16 */
public StackOfIntegers() {
this (DEFAULT_CAPACITY);
}
/** Construct a stack with the specified maximum capacity */
public StackOfIntegers(int capacity) {
elements = new int[capacity];
}
max capacity 16
380 Chapter 10
Object-Oriented Thinking
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
double the capacity
add to stack
/** Push a new integer to the top of the stack */
public void push(int value) {
if (size >= elements.length) {
int[] temp = new int[elements.length * 2];
System.arraycopy(elements, 0, temp, 0, elements.length);
elements = temp;
}
elements[size++] = value;
}
/** Return and remove the top element from the stack */
public int pop() {
return elements[——size];
}
/** Return the top element from the stack */
public int peek() {
return elements[size - 1];
}
/** Test whether the stack is empty */
public boolean empty() {
return size == 0;
}
/** Return the number of elements in the stack */
public int getSize() {
return size;
}
}
10.7 Processing Primitive Data Type Values as Objects
Key
Point
why wrapper class?
A primitive type value is not an object, but it can be wrapped in an object using a
wrapper class in the Java API.
Owing to performance considerations, primitive data type values are not objects in Java.
Because of the overhead of processing objects, the language’s performance would be
adversely affected if primitive data type values were treated as objects. However, many Java
methods require the use of objects as arguments. Java offers a convenient way to incorporate,
or wrap, a primitive data type into an object (e.g., wrapping int into the Integer class,
wrapping double into the Double class, and wrapping char into the Character class,). By
using a wrapper class, you can process primitive data type values as objects. Java provides
Boolean, Character, Double, Float, Byte, Short, Integer, and Long wrapper classes
in the java.lang package for primitive data types. The Boolean class wraps a Boolean
value true or false. This section uses Integer and Double as examples to introduce the
numeric wrapper classes.
Note
naming convention
Most wrapper class names for a primitive type are the same as the primitive data type
name with the first letter capitalized. The exceptions are Integer and Character.
Numeric wrapper classes are very similar to each other. Each contains the methods
doubleValue(), floatValue(), intValue(), longValue(), shortValue(), and
byteValue(). These methods “convert” objects into primitive type values. The key features
of Integer and Double are shown in Figure 10.14.
10.7 Processing Primitive Data Type Values as Objects 381
java.lang.Integer
java.lang.Double
-value: int
-value: double
+MAX_VALUE: int
+MAX_VALUE: double
+MIN_VALUE: int
+MIN_VALUE: double
+Integer(value: int)
+Double(value: double)
+Integer(s: String)
+Double(s: String)
+byteValue(): byte
+byteValue(): byte
+shortValue(): short
+shortValue(): short
+intValue(): int
+intValue(): int
+longValue(): long
+longValue(): long
+floatValue(): float
+floatValue(): float
+doubleValue(): double
+doubleValue(): double
+compareTo(o: Integer): int
+compareTo(o: Double): int
+toString(): String
+toString(): String
+valueOf(s: String): Integer
+valueOf(s: String): Double
+valueOf(s: String, radix: int): Integer
+valueOf(s: String, radix: int): Double
+parseInt(s: String): int
+parseDouble(s: String): double
+parseInt(s: String, radix: int): int
+parseDouble(s: String, radix: int): double
FIGURE 10.14 The wrapper classes provide constructors, constants, and conversion methods for manipulating various
data types.
You can construct a wrapper object either from a primitive data type value or from a string
representing the numeric value—for example, new Double(5.0), new Double("5.0"),
new Integer(5), and new Integer("5").
The wrapper classes do not have no-arg constructors. The instances of all wrapper classes
are immutable; this means that, once the objects are created, their internal values cannot be
changed.
Each numeric wrapper class has the constants MAX_VALUE and MIN_VALUE. MAX_VALUE
represents the maximum value of the corresponding primitive data type. For Byte, Short,
Integer, and Long, MIN_VALUE represents the minimum byte, short, int, and long
values. For Float and Double, MIN_VALUE represents the minimum positive float and
double values. The following statements display the maximum integer (2,147,483,647),
the minimum positive float (1.4E–45), and the maximum double floating-point number
(1.79769313486231570e + 308d).
constructors
no no-arg constructor
immutable
constants
System.out.println("The maximum integer is " + Integer.MAX_VALUE);
System.out.println("The minimum positive float is " +
Float.MIN_VALUE);
System.out.println(
"The maximum double-precision floating-point number is " +
Double.MAX_VALUE);
Each numeric wrapper class contains the methods doubleValue(), floatValue(),
intValue(), longValue(), and shortValue() for returning a double, float, int,
long, or short value for the wrapper object. For example,
conversion methods
new Double(12.4).intValue() returns 12;
new Integer(12).doubleValue() returns 12.0;
Recall that the String class contains the compareTo method for comparing two strings.
The numeric wrapper classes contain the compareTo method for comparing two numbers
compareTo method
382 Chapter 10
Object-Oriented Thinking
and returns 1, 0, or -1, if this number is greater than, equal to, or less than the other number.
For example,
new Double(12.4).compareTo(new Double(12.3)) returns 1;
new Double(12.3).compareTo(new Double(12.3)) returns 0;
new Double(12.3).compareTo(new Double(12.51)) returns -1;
static valueOf methods
The numeric wrapper classes have a useful static method, valueOf (String s). This
method creates a new object initialized to the value represented by the specified string. For
example,
Double doubleObject = Double.valueOf("12.4");
Integer integerObject = Integer.valueOf("12");
static parsing methods
You have used the parseInt method in the Integer class to parse a numeric string into
an int value and the parseDouble method in the Double class to parse a numeric string into
a double value. Each numeric wrapper class has two overloaded parsing methods to parse a
numeric string into an appropriate numeric value based on 10 (decimal) or any specified radix
(e.g., 2 for binary, 8 for octal, and 16 for hexadecimal).
// These two methods are in the Byte class
public static byte parseByte(String s)
public static byte parseByte(String s, int radix)
// These two methods are in the Short class
public static short parseShort(String s)
public static short parseShort(String s, int radix)
// These two methods are in the Integer class
public static int parseInt(String s)
public static int parseInt(String s, int radix)
// These two methods are in the Long class
public static long parseLong(String s)
public static long parseLong(String s, int radix)
// These two methods are in the Float class
public static float parseFloat(String s)
public static float parseFloat(String s, int radix)
// These two methods are in the Double class
public static double parseDouble(String s)
public static double parseDouble(String s, int radix)
For example,
Integer.parseInt("11",
Integer.parseInt("12",
Integer.parseInt("13",
Integer.parseInt("1A",
converting decimal to hex
2) returns 3;
8) returns 10;
10) returns 13;
16) returns 26;
Integer.parseInt("12", 2) would raise a runtime exception because 12 is not a
binary number.
Note that you can convert a decimal number into a hex number using the format method.
For example,
String.format("%x", 26) returns 1A;
10.8 Automatic Conversion between Primitive Types and Wrapper Class Types 383
10.7 Describe primitive-type wrapper classes.
10.8 Can each of the following statements be compiled?
✓
Check
Point
a. Integer i = new Integer("23");
b. Integer i = new Integer(23);
c. Integer i = Integer.valueOf("23");
d. Integer i = Integer.parseInt("23", 8);
e. Double d = new Double();
f. Double d = Double.valueOf("23.45");
g. int i = (Integer.valueOf("23")).intValue();
h. double d = (Double.valueOf("23.4")).doubleValue();
i. int i = (Double.valueOf("23.4")).intValue();
j. String s = (Double.valueOf("23.4")).toString();
10.9 How do you convert an integer into a string? How do you convert a numeric string
into an integer? How do you convert a double number into a string? How do you
convert a numeric string into a double value?
10.10 Show the output of the following code.
public class Test {
public static void main(String[] args) {
Integer x = new Integer(3);
System.out.println(x.intValue());
System.out.println(x.compareTo(new Integer(4)));
}
}
10.11 What is the output of the following code?
public class Test {
public static void main(String[] args) {
System.out.println(Integer.parseInt("10"));
System.out.println(Integer.parseInt("10", 10));
System.out.println(Integer.parseInt("10", 16));
System.out.println(Integer.parseInt("11"));
System.out.println(Integer.parseInt("11", 10));
System.out.println(Integer.parseInt("11", 16));
}
}
10.8 Automatic Conversion between Primitive Types
and Wrapper Class Types
A primitive type value can be automatically converted to an object using a wrapper
class, and vice versa, depending on the context.
Converting a primitive value to a wrapper object is called boxing. The reverse conversion is
called unboxing. Java allows primitive types and wrapper classes to be converted automatically. The compiler will automatically box a primitive value that appears in a context requiring an object, and will unbox an object that appears in a context requiring a primitive value.
This is called autoboxing and autounboxing.
Key
Point
boxing
unboxing
autoboxing
autounboxing
384 Chapter 10
Object-Oriented Thinking
For instance, the following statement in (a) can be simplified as in (b) due to autoboxing.
Integer intObject = new Integer (2);
Equivalent
(a)
Integer intObject = 2;
autoboxing
(b)
Consider the following example:
1
2
Integer[] intArray = {1, 2, 3};
System.out.println(intArray[0] + intArray[1] + intArray[2]);
In line 1, the primitive values 1, 2, and 3 are automatically boxed into objects new Integer(1),
new Integer(2), and new Integer(3). In line 2, the objects intArray[0], intArray[1],
and intArray[2] are automatically unboxed into int values that are added together.
✓
Check
Point
10.12 What are autoboxing and autounboxing? Are the following statements correct?
a.
b.
c.
d.
e.
f.
Integer x = 3 + new Integer(5);
Integer x = 3;
Double x = 3;
Double x = 3.0;
int x = new Integer(3);
int x = new Integer(3) + new Integer(4);
10.13 Show the output of the following code?
public class Test {
public static void main(String[] args) {
Double x = 3.5;
System.out.println(x.intValue());
System.out.println(x.compareTo(4.5));
}
}
10.9 The BigInteger and BigDecimal Classes
Key
Point
immutable
VideoNote
Process large numbers
The BigInteger and BigDecimal classes can be used to represent integers or
decimal numbers of any size and precision.
If you need to compute with very large integers or high-precision floating-point values, you can use the BigInteger and BigDecimal classes in the java.math package. Both are immutable. The largest integer of the long type is Long.MAX_VALUE (i.e.,
9223372036854775807). An instance of BigInteger can represent an integer of any size.
You can use new BigInteger(String) and new BigDecimal(String) to create an
instance of BigInteger and BigDecimal, use the add, subtract, multiply, divide,
and remainder methods to perform arithmetic operations, and use the compareTo method
to compare two big numbers. For example, the following code creates two BigInteger
objects and multiplies them.
BigInteger a = new BigInteger("9223372036854775807");
BigInteger b = new BigInteger("2");
BigInteger c = a.multiply(b); // 9223372036854775807 * 2
System.out.println(c);
10.9 The BigInteger and BigDecimal Classes 385
The output is 18446744073709551614.
There is no limit to the precision of a BigDecimal object. The divide method may throw
an ArithmeticException if the result cannot be terminated. However, you can use the
overloaded divide(BigDecimal d, int scale, int roundingMode) method to specify a scale and a rounding mode to avoid this exception, where scale is the maximum number
of digits after the decimal point. For example, the following code creates two BigDecimal
objects and performs division with scale 20 and rounding mode BigDecimal.ROUND_UP.
BigDecimal a = new BigDecimal(1.0);
BigDecimal b = new BigDecimal(3);
BigDecimal c = a.divide(b, 20, BigDecimal.ROUND_UP);
System.out.println(c);
The output is 0.33333333333333333334.
Note that the factorial of an integer can be very large. Listing 10.9 gives a method that can
return the factorial of any integer.
LISTING 10.9 LargeFactorial.java
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
import java.math.*;
public class LargeFactorial {
public static void main(String[] args) {
System.out.println("50! is \n" + factorial(50));
}
public static BigInteger factorial(long n) {
BigInteger result = BigInteger.ONE;
for (int i = 1; i <= n; i++)
result = result.multiply(new BigInteger(i + ""));
constant
multiply
return result;
}
}
50! is
30414093201713378043612608166064768844377641568960512000000000000
BigInteger.ONE (line 9) is a constant defined in the BigInteger class. BigInteger.ONE
is the same as new BigInteger("1").
A new result is obtained by invoking the multiply method (line 11).
10.14 What is the output of the following code?
public class Test {
public static void main(String[] args) {
java.math.BigInteger x = new java.math.BigInteger("3");
java.math.BigInteger y = new java.math.BigInteger("7");
java.math.BigInteger z = x.add(y);
System.out.println("x is " + x);
System.out.println("y is " + y);
System.out.println("z is " + z);
}
}
✓
Check
Point
386 Chapter 10
Object-Oriented Thinking
10.10 The String Class
Key
Point
VideoNote
The String class
A String object is immutable: Its content cannot be changed once the string is
created.
Strings were introduced in Section 4.4. You know strings are objects. You can invoke the
charAt(index) method to obtain a character at the specified index from a string, the
length() method to return the size of a string, the substring method to return a substring
in a string, and the indexOf and lastIndexOf methods to return the first or last index of a
matching character or a substring. We will take a closer look at strings in this section.
The String class has 13 constructors and more than 40 methods for manipulating strings.
Not only is it very useful in programming, but it is also a good example for learning classes
and objects.
10.10.1
Constructing a String
You can create a string object from a string literal or from an array of characters. To create a
string from a string literal, use the syntax:
String newString = new String(stringLiteral);
The argument stringLiteral is a sequence of characters enclosed inside double quotes.
The following statement creates a String object message for the string literal "Welcome
to Java":
String message = new String("Welcome to Java");
string literal object
Java treats a string literal as a String object. Thus, the following statement is valid:
String message = "Welcome to Java";
You can also create a string from an array of characters. For example, the following statements create the string "Good Day":
char[] charArray = {'G', 'o', 'o', 'd', ' ', 'D', 'a', 'y'};
String message = new String(charArray);
Note
A String variable holds a reference to a String object that stores a string value.
Strictly speaking, the terms String variable, String object, and string value are
different, but most of the time the distinctions between them can be ignored. For simplicity, the term string will often be used to refer to String variable, String object,
and string value.
String variable, String
object, string value
10.10.2
immutable
Immutable Strings and Interned Strings
A String object is immutable; its contents cannot be changed. Does the following code
change the contents of the string?
String s = "Java";
s = "HTML";
The answer is no. The first statement creates a String object with the content "Java" and
assigns its reference to s. The second statement creates a new String object with the content
"HTML" and assigns its reference to s. The first String object still exists after the assignment, but it can no longer be accessed, because variable s now points to the new object, as
shown in Figure 10.15.
10.10 The String Class 387
After executing String s = "Java";
s
: String
After executing s = "HTML";
: String
s
String object for "Java"
String object for "Java"
Contents cannot be changed
This string object is
now unreferenced
: String
String object for "HTML"
FIGURE 10.15 Strings are immutable; once created, their contents cannot be changed.
Because strings are immutable and are ubiquitous in programming, the JVM uses a unique
instance for string literals with the same character sequence in order to improve efficiency
and save memory. Such an instance is called an interned string. For example, the following
statements:
String s1 = "Welcome to Java";
s1
s3
String s2 = new String("Welcome to Java");
interned string
: String
Interned string object for
"Welcome to Java"
String s3 = "Welcome to Java";
System.out.println("s1 == s2 is " + (s1 == s2)); s2
System.out.println("s1 == s3 is " + (s1 == s3));
: String
A string object for
"Welcome to Java"
display
s1 == s2 is false
s1 == s3 is true
In the preceding statements, s1 and s3 refer to the same interned string—"Welcome to
Java"—so s1 == s3 is true. However, s1 == s2 is false, because s1 and s2 are two
different string objects, even though they have the same contents.
10.10.3
Replacing and Splitting Strings
The String class provides the methods for replacing and splitting strings, as shown in
Figure 10.16.
java.lang.String
+replace(oldChar: char,
newChar: char): String
Returns a new string that replaces all matching characters in this
string with the new character.
+replaceFirst(oldString: String,
newString: String): String
Returns a new string that replaces the first matching substring in
this string with the new substring.
+replaceAll(oldString: String,
newString: String): String
Returns a new string that replaces all matching substrings in this
string with the new substring.
+split(delimiter: String):
String[]
Returns an array of strings consisting of the substrings split by the
delimiter.
FIGURE 10.16 The String class contains the methods for replacing and splitting strings.
388 Chapter 10
Object-Oriented Thinking
Once a string is created, its contents cannot be changed. The methods replace,
replaceFirst, and replaceAll return a new string derived from the original string
(without changing the original string!). Several versions of the replace methods are provided to replace a character or a substring in the string with a new character or a new
substring.
For example,
replace
replaceFirst
replace
replace
split
"Welcome".replace('e', 'A') returns a new string, WAlcomA.
"Welcome".replaceFirst("e", "AB") returns a new string, WABlcome.
"Welcome".replace("e", "AB") returns a new string, WABlcomAB.
"Welcome".replace("el", "AB") returns a new string, WABcome.
The split method can be used to extract tokens from a string with the specified delimiters.
For example, the following code
String[] tokens = "Java#HTML#Perl".split("#");
for (int i = 0; i < tokens.length; i++)
System.out.print(tokens[i] + " ");
displays
Java HTML Perl
10.10.4
why regular expression?
regular expression
regex
matches(regex)
Matching, Replacing and Splitting by Patterns
Often you will need to write code that validates user input, such as to check whether the input
is a number, a string with all lowercase letters, or a Social Security number. How do you write
this type of code? A simple and effective way to accomplish this task is to use the regular
expression.
A regular expression (abbreviated regex) is a string that describes a pattern for matching
a set of strings. You can match, replace, or split a string by specifying a pattern. This is an
extremely useful and powerful feature.
Let us begin with the matches method in the String class. At first glance, the matches
method is very similar to the equals method. For example, the following two statements both
evaluate to true.
"Java".matches("Java");
"Java".equals("Java");
However, the matches method is more powerful. It can match not only a fixed string, but
also a set of strings that follow a pattern. For example, the following statements all evaluate
to true:
"Java is fun".matches("Java.*")
"Java is cool".matches("Java.*")
"Java is powerful".matches("Java.*")
Java.* in the preceding statements is a regular expression. It describes a string pattern that
begins with Java followed by any zero or more characters. Here, the substring matches any
zero or more characters.
The following statement evaluates to true.
"440-02-4534".matches("\\d{3}-\\d{2}-\\d{4}")
Here \\d represents a single digit, and \\d{3} represents three digits.
10.10 The String Class 389
The replaceAll, replaceFirst, and split methods can be used with a regular
expression. For example, the following statement returns a new string that replaces $, +, or #
in a+b$#c with the string NNN.
String s = "a+b$#c".replaceAll("[$+#]", "NNN");
System.out.println(s);
replaceAll(regex)
Here the regular expression [$+#] specifies a pattern that matches $, +, or #. So, the output
is aNNNbNNNNNNc.
The following statement splits the string into an array of strings delimited by punctuation marks.
String[] tokens = "Java,C?C#,C++".split("[.,:;?]");
split(regex)
for (int i = 0; i < tokens.length; i++)
System.out.println(tokens[i]);
In this example, the regular expression [.,:;?] specifies a pattern that matches ., ,, :, ;, or
?. Each of these characters is a delimiter for splitting the string. Thus, the string is split into
Java, C, C#, and C++, which are stored in array tokens.
Regular expression patterns are complex for beginning students to understand. For this
reason, simple patterns are introduced in this section. Please refer to Appendix H, Regular
Expressions, to learn more about these patterns.
10.10.5
further studies
Conversion between Strings and Arrays
Strings are not arrays, but a string can be converted into an array, and vice versa. To convert a
string into an array of characters, use the toCharArray method. For example, the following
statement converts the string Java to an array.
toCharArray
char[] chars = "Java".toCharArray();
Thus, chars[0] is J, chars[1] is a, chars[2] is v, and chars[3] is a.
You can also use the getChars(int srcBegin, int srcEnd, char[] dst,
int dstBegin) method to copy a substring of the string from index srcBegin to index
srcEnd-1 into a character array dst starting from index dstBegin. For example, the following code copies a substring "3720" in "CS3720" from index 2 to index 6-1 into the
character array dst starting from index 4.
char[] dst = {'J', 'A', 'V', 'A', '1', '3', '0', '1'};
"CS3720".getChars(2, 6, dst, 4);
getChars
Thus, dst becomes {'J', 'A', 'V', 'A', '3', '7', '2', '0'}.
To convert an array of characters into a string, use the String(char[]) constructor or
the valueOf(char[]) method. For example, the following statement constructs a string
from an array using the String constructor.
String str = new String(new char[]{'J', 'a', 'v', 'a'});
The next statement constructs a string from an array using the valueOf method.
String str = String.valueOf(new char[]{'J', 'a', 'v', 'a'});
10.10.6
Converting Characters and Numeric Values to Strings
Recall that you can use Double.parseDouble(str) or Integer.parseInt(str) to
convert a string to a double value or an int value and you can convert a character or a
number into a string by using the string concatenating operator. Another way of converting a
valueOf
390 Chapter 10
Object-Oriented Thinking
number into a string is to use the overloaded static valueOf method. This method can also be
used to convert a character or an array of characters into a string, as shown in Figure 10.17.
overloaded valueOf
java.lang.String
+valueOf(c: char): String
Returns a string consisting of the character c.
+valueOf(data: char[]): String
Returns a string consisting of the characters in the array.
+valueOf(d: double): String
Returns a string representing the double value.
+valueOf(f: float): String
Returns a string representing the float value.
+valueOf(i: int): String
Returns a string representing the int value.
+valueOf(l: long): String
Returns a string representing the long value.
+valueOf(b: boolean): String
Returns a string representing the boolean value.
FIGURE 10.17 The String class contains the static methods for creating strings from primitive type values.
For example, to convert a double value 5.44 to a string, use String.valueOf(5.44).
The return value is a string consisting of the characters '5', '.', '4', and '4'.
10.10.7
Formatting Strings
The String class contains the static format method to return a formatted string. The syntax
to invoke this method is:
String.format(format, item1, item2, ..., itemk)
This method is similar to the printf method except that the format method returns a formatted string, whereas the printf method displays a formatted string. For example,
String s = String.format("%7.2f%6d%-4s", 45.556, 14, "AB");
System.out.println(s);
displays
45.56
14AB
Note that
System.out.printf(format, item1, item2, ..., itemk);
is equivalent to
System.out.print(
String.format(format, item1, item2, ..., itemk));
where the square box ( ) denotes a blank space.
✓
Check
Point
10.15 Suppose that s1, s2, s3, and s4 are four strings, given as follows:
String
String
String
String
s1
s2
s3
s4
=
=
=
=
"Welcome to Java";
s1;
new String("Welcome to Java");
"Welcome to Java";
What are the results of the following expressions?
a.
b.
s1 == s2
s1 == s3
10.10 The String Class 391
c. s1 == s4
d. s1.equals(s3)
e. s1.equals(s4)
f. "Welcome to Java".replace("Java", "HTML")
g. s1.replace('o', 'T')
h. s1.replaceAll("o", "T")
i. s1.replaceFirst("o", "T")
j.
s1.toCharArray()
10.16 To create the string Welcome
to Java, you may use a statement like this:
String s = "Welcome to Java";
or:
String s = new String("Welcome to Java");
Which one is better? Why?
10.17 What is the output of the following code?
String s1 = "Welcome to Java";
String s2 = s1.replace("o", "abc");
System.out.println(s1);
System.out.println(s2);
10.18 Let
s1 be "Welcome" and s2 be "welcome". Write the code for the following
statements:
a. Replace all occurrences of the character e with E in s1 and assign the new string
to s2.
b. Split Welcome to Java and HTML into an array tokens delimited by a space
and assign the first two tokens into s1 and s2.
10.19
10.20
10.21
10.22
Does any method in the String class change the contents of the string?
Suppose string s is created using new String(); what is s.length()?
How do you convert a char, an array of characters, or a number to a string?
Why does the following code cause a NullPointerException?
1
2
3
4
5
6
7
8
9
10
11
12
public class Test {
private String text;
public Test(String s) {
String text = s;
}
public static void main(String[] args) {
Test test = new Test("ABC");
System.out.println(test.text.toLowerCase());
}
}
10.23 What is wrong in the following program?
1
2
3
public class Test {
String text;
392 Chapter 10
Object-Oriented Thinking
4
5
6
7
8
9
10
11
12
public void Test(String s) {
text = s;
}
public static void main(String[] args) {
Test test = new Test("ABC");
System.out.println(test);
}
}
10.24 Show the output of the following code.
public class Test {
public static void main(String[] args) {
System.out.println("Hi, ABC, good".matches("ABC "));
System.out.println("Hi, ABC, good".matches(".*ABC.*"));
System.out.println("A,B;C".replaceAll(",;", "#"));
System.out.println("A,B;C".replaceAll("[,;]", "#"));
String[] tokens = "A,B;C".split("[,;]");
for (int i = 0; i < tokens.length; i++)
System.out.print(tokens[i] + " ");
}
}
10.25 Show the output of the following code.
public class Test {
public static void main(String[] args) {
String s = "Hi, Good Morning";
System.out.println(m(s));
}
public static int m(String s) {
int count = 0;
for (int i = 0; i < s.length(); i++)
if (Character.isUpperCase(s.charAt(i)))
count++;
return count;
}
}
10.11 The StringBuilder and StringBuffer Classes
Key
Point
StringBuilder
The StringBuilder and StringBuffer classes are similar to the String class
except that the String class is immutable.
In general, the StringBuilder and StringBuffer classes can be used wherever a string
is used. StringBuilder and StringBuffer are more flexible than String. You can add,
insert, or append new contents into StringBuilder and StringBuffer objects, whereas
the value of a String object is fixed once the string is created.
The StringBuilder class is similar to StringBuffer except that the methods for modifying the buffer in StringBuffer are synchronized, which means that only one task is
allowed to execute the methods. Use StringBuffer if the class might be accessed by multiple tasks concurrently, because synchronization is needed in this case to prevent corruptions to
10.11 The StringBuilder and StringBuffer Classes 393
StringBuffer. Concurrent programming will be introduced in Chapter 30. Using StringBuilder is more efficient if it is accessed by just a single task, because no synchronization is
needed in this case. The constructors and methods in StringBuffer and StringBuilder
are almost the same. This section covers StringBuilder. You can replace StringBuilder
in all occurrences in this section by StringBuffer. The program can compile and run with-
out any other changes.
The StringBuilder class has three constructors and more than 30 methods for managing
the builder and modifying strings in the builder. You can create an empty string builder or a
string builder from a string using the constructors, as shown in Figure 10.18.
StringBuilder constructors
java.lang.StringBuilder
+StringBuilder()
Constructs an empty string builder with capacity 16.
+StringBuilder(capacity: int)
Constructs a string builder with the specified capacity.
+StringBuilder(s: String)
Constructs a string builder with the specified string.
FIGURE 10.18 The StringBuilder class contains the constructors for creating instances
of StringBuilder.
10.11.1
Modifying Strings in the StringBuilder
You can append new contents at the end of a string builder, insert new contents at a specified position in a string builder, and delete or replace characters in a string builder, using the
methods listed in Figure 10.19.
java.lang.StringBuilder
+append(data: char[]): StringBuilder
+append(data: char[], offset: int, len: int):
StringBuilder
Appends a char array into this string builder.
Appends a subarray in data into this string builder.
+append(v: aPrimitiveType): StringBuilder
Appends a primitive type value as a string to this
builder.
+append(s: String): StringBuilder
Appends a string to this string builder.
+delete(startIndex: int, endIndex: int):
StringBuilder
+deleteCharAt(index: int): StringBuilder
Deletes characters from startIndex to endIndex-1.
+insert(index: int, data: char[], offset: int,
len: int): StringBuilder
+insert(offset: int, data: char[]):
StringBuilder
+insert(offset: int, b: aPrimitiveType):
StringBuilder
Inserts a subarray of the data in the array into the builder
at the specified index.
Inserts data into this builder at the position offset.
+insert(offset: int, s: String): StringBuilder
+replace(startIndex: int, endIndex: int, s:
String): StringBuilder
+reverse(): StringBuilder
+setCharAt(index: int, ch: char): void
Deletes a character at the specified index.
Inserts a value converted to a string into this builder.
Inserts a string into this builder at the position offset.
Replaces the characters in this builder from startIndex
to endIndex-1 with the specified string.
Reverses the characters in the builder.
Sets a new character at the specified index in this
builder.
FIGURE 10.19 The StringBuilder class contains the methods for modifying string builders.
394 Chapter 10
Object-Oriented Thinking
The StringBuilder class provides several overloaded methods to append boolean,
char, char[], double, float, int, long, and String into a string builder. For example,
the following code appends strings and characters into stringBuilder to form a new string,
Welcome to Java.
append
StringBuilder stringBuilder = new StringBuilder();
stringBuilder.append("Welcome");
stringBuilder.append(' ');
stringBuilder.append("to");
stringBuilder.append(' ');
stringBuilder.append("Java");
The StringBuilder class also contains overloaded methods to insert boolean, char, char
array, double, float, int, long, and String into a string builder. Consider the following
code:
insert
stringBuilder.insert(11, "HTML and ");
Suppose stringBuilder contains Welcome to Java before the insert method is applied.
This code inserts "HTML and " at position 11 in stringBuilder (just before the J). The
new stringBuilder is Welcome to HTML and Java.
You can also delete characters from a string in the builder using the two delete methods,
reverse the string using the reverse method, replace characters using the replace method,
or set a new character in a string using the setCharAt method.
For example, suppose stringBuilder contains Welcome to Java before each of the
following methods is applied:
delete
deleteCharAt
reverse
replace
setCharAt
stringBuilder.delete(8, 11) changes the builder to Welcome Java.
stringBuilder.deleteCharAt(8) changes the builder to Welcome o Java.
stringBuilder.reverse() changes the builder to avaJ ot emocleW.
stringBuilder.replace(11, 15, "HTML") changes the builder to Welcome to HTML.
stringBuilder.setCharAt(0, 'w') sets the builder to welcome to Java.
All these modification methods except setCharAt do two things:
ignore return value
■
Change the contents of the string builder
■
Return the reference of the string builder
For example, the following statement
StringBuilder stringBuilder1 = stringBuilder.reverse();
reverses the string in the builder and assigns the builder’s reference to stringBuilder1.
Thus, stringBuilder and stringBuilder1 both point to the same StringBuilder
object. Recall that a value-returning method can be invoked as a statement, if you are not
interested in the return value of the method. In this case, the return value is simply ignored.
For example, in the following statement
stringBuilder.reverse();
the return value is ignored.
Tip
String or StringBuilder?
If a string does not require any change, use String rather than StringBuilder. Java
can perform some optimizations for String, such as sharing interned strings.
10.11 The StringBuilder and StringBuffer Classes 395
10.11.2
The toString, capacity, length, setLength,
and charAt Methods
The StringBuilder class provides the additional methods for manipulating a string builder
and obtaining its properties, as shown in Figure 10.20.
java.lang.StringBuilder
+toString(): String
+capacity(): int
Returns a string object from the string builder.
Returns the capacity of this string builder.
+charAt(index: int): char
Returns the character at the specified index.
+length(): int
+setLength(newLength: int): void
Returns the number of characters in this builder.
Sets a new length in this builder.
+substring(startIndex: int): String
Returns a substring starting at startIndex.
+substring(startIndex: int, endIndex: int):
String
+trimToSize(): void
Returns a substring from startIndex to endIndex-1.
Reduces the storage size used for the string builder.
FIGURE 10.20 The StringBuilder class contains the methods for modifying string builders.
The capacity() method returns the current capacity of the string builder. The capacity is
the number of characters the string builder is able to store without having to increase its size.
The length() method returns the number of characters actually stored in the string
builder. The setLength(newLength) method sets the length of the string builder. If the
newLength argument is less than the current length of the string builder, the string builder is
truncated to contain exactly the number of characters given by the newLength argument. If
the newLength argument is greater than or equal to the current length, sufficient null characters (\u0000) are appended to the string builder so that length becomes the newLength
argument. The newLength argument must be greater than or equal to 0.
The charAt(index) method returns the character at a specific index in the string
builder. The index is 0 based. The first character of a string builder is at index 0, the next at
index 1, and so on. The index argument must be greater than or equal to 0, and less than the
length of the string builder.
capacity()
length()
setLength(int)
charAt(int)
Note
The length of the string is always less than or equal to the capacity of the builder. The
length is the actual size of the string stored in the builder, and the capacity is the current
size of the builder. The builder’s capacity is automatically increased if more characters
are added to exceed its capacity. Internally, a string builder is an array of characters, so
the builder’s capacity is the size of the array. If the builder’s capacity is exceeded, the
array is replaced by a new array. The new array size is 2 * (the previous array
size + 1).
length and capacity
Tip
You can use new StringBuilder(initialCapacity) to create a StringBuilder with a specified initial capacity. By carefully choosing the initial capacity, you
can make your program more efficient. If the capacity is always larger than the actual
length of the builder, the JVM will never need to reallocate memory for the builder. On
the other hand, if the capacity is too large, you will waste memory space. You can use
the trimToSize() method to reduce the capacity to the actual size.
initial capacity
trimToSize()
396 Chapter 10
Object-Oriented Thinking
10.11.3
Case Study: Ignoring Nonalphanumeric Characters When
Checking Palindromes
Listing 5.14, Palindrome.java, considered all the characters in a string to check whether it
is a palindrome. Write a new program that ignores nonalphanumeric characters in checking
whether a string is a palindrome.
Here are the steps to solve the problem:
1. Filter the string by removing the nonalphanumeric characters. This can be done by
creating an empty string builder, adding each alphanumeric character in the string
to a string builder, and returning the string from the string builder. You can use the
isLetterOrDigit(ch) method in the Character class to check whether character
ch is a letter or a digit.
2. Obtain a new string that is the reversal of the filtered string. Compare the reversed string
with the filtered string using the equals method.
The complete program is shown in Listing 10.10.
LISTING 10.10 PalindromeIgnoreNonAlphanumeric.java
check palindrome
add letter or digit
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
import java.util.Scanner;
public class PalindromeIgnoreNonAlphanumeric {
/** Main method */
public static void main(String[] args) {
// Create a Scanner
Scanner input = new Scanner(System.in);
// Prompt the user to enter a string
System.out.print("Enter a string: ");
String s = input.nextLine();
// Display result
System.out.println("Ignoring nonalphanumeric characters, \nis "
+ s + " a palindrome? " + isPalindrome(s));
}
/** Return true if a string is a palindrome */
public static boolean isPalindrome(String s) {
// Create a new string by eliminating nonalphanumeric chars
String s1 = filter(s);
// Create a new string that is the reversal of s1
String s2 = reverse(s1);
// Check if the reversal is the same as the original string
return s2.equals(s1);
}
/** Create a new string by eliminating nonalphanumeric chars */
public static String filter(String s) {
// Create a string builder
StringBuilder stringBuilder = new StringBuilder();
// Examine each char in the string to skip alphanumeric char
for (int i = 0; i < s.length(); i++) {
if (Character.isLetterOrDigit(s.charAt(i))) {
stringBuilder.append(s.charAt(i));
}
10.11 The StringBuilder and StringBuffer Classes 397
40
41
42
43
44
45
46
47
48
49
50
51
52
}
// Return a new filtered string
return stringBuilder.toString();
}
/** Create a new string by reversing a specified string */
public static String reverse(String s) {
StringBuilder stringBuilder = new StringBuilder(s);
stringBuilder.reverse(); // Invoke reverse in StringBuilder
return stringBuilder.toString();
}
}
Enter a string: ab<c>cb?a
Ignoring nonalphanumeric characters,
is ab<c>cb?a a palindrome? true
Enter a string: abcc><?cab
Ignoring nonalphanumeric characters,
is abcc><?cab a palindrome? false
The filter(String s) method (lines 31–44) examines each character in string s and copies it to a string builder if the character is a letter or a numeric character. The filter method
returns the string in the builder. The reverse(String s) method (lines 47–51) creates a
new string that reverses the specified string s. The filter and reverse methods both return
a new string. The original string is not changed.
The program in Listing 5.14 checks whether a string is a palindrome by comparing pairs
of characters from both ends of the string. Listing 10.10 uses the reverse method in the
StringBuilder class to reverse the string, then compares whether the two strings are equal
to determine whether the original string is a palindrome.
10.26 What is the difference between StringBuilder and StringBuffer?
10.27 How do you create a string builder from a string? How do you return a string from a
string builder?
10.28 Write three statements to reverse a string
s using the reverse method in the
StringBuilder class.
10.29 Write three statements to delete a substring from a string s of 20 characters, starting at index 4 and ending with index 10. Use the delete method in the StringBuilder class.
10.30 What is the internal storage for characters in a string and a string builder?
10.31 Suppose that s1 and s2 are given as follows:
StringBuilder s1 = new StringBuilder("Java");
StringBuilder s2 = new StringBuilder("HTML");
Show the value of s1 after each of the following statements. Assume that the statements are independent.
a. s1.append(" is fun");
b. s1.append(s2);
✓
Check
Point
398 Chapter 10
Object-Oriented Thinking
c.
d.
e.
f.
g.
h.
i.
s1.insert(2, "is fun");
s1.insert(1, s2);
s1.charAt(2);
s1.length();
s1.deleteCharAt(3);
s1.delete(1, 3);
s1.reverse();
j. s1.replace(1,
3, "Computer");
k. s1.substring(1,
3);
l. s1.substring(2);
10.32 Show the output of the following program:
public class Test {
public static void main(String[] args) {
String s = "Java";
StringBuilder builder = new StringBuilder(s);
change(s, builder);
System.out.println(s);
System.out.println(builder);
}
private static void change(String s, StringBuilder builder) {
s = s + " and HTML";
builder.append(" and HTML");
}
}
KEY TERMS
abstract data type (ADT) 366
aggregation 374
boxing 383
class abstraction 366
class encapsulation 366
class’s contract 366
composition 374
has-a relationship 374
multiplicity 373
stack 378
unboxing 383
CHAPTER SUMMARY
1. The procedural paradigm focuses on designing methods. The object-oriented paradigm
couples data and methods together into objects. Software design using the objectoriented paradigm focuses on objects and operations on objects. The object-oriented
approach combines the power of the procedural paradigm with an added dimension that
integrates data with operations into objects.
2. Many Java methods require the use of objects as arguments. Java offers a convenient
way to incorporate, or wrap, a primitive data type into an object (e.g., wrapping int into
the Integer class, and wrapping double into the Double class).
Programming Exercises 399
3. Java can automatically convert a primitive type value to its corresponding wrapper
object in the context and vice versa.
4. The BigInteger class is useful for computing and processing integers of any size. The
BigDecimal class can be used to compute and process floating-point numbers with any
arbitrary precision.
5. A String object is immutable; its contents cannot be changed. To improve efficiency
and save memory, the JVM stores two literal strings that have the same character
sequence in a unique object. This unique object is called an interned string object.
6. A regular expression (abbreviated regex) is a string that describes a pattern for matching a set of strings. You can match, replace, or split a string by specifying a pattern.
7. The StringBuilder and StringBuffer classes can be used to replace the String
class. The String object is immutable, but you can add, insert, or append new contents
into StringBuilder and StringBuffer objects. Use String if the string contents
do not require any change, and use StringBuilder or StringBuffer if they might
change.
QUIZ
Answer the quiz for this chapter online at www.cs.armstrong.edu/liang/intro10e/quiz.html.
PROGRAMMING EXERCISES
Sections 10.2–10.3
*10.1 (The Time class) Design a class named Time. The class contains:
■
■
■
■
■
■
The data fields hour, minute, and second that represent a time.
A no-arg constructor that creates a Time object for the current time. (The
values of the data fields will represent the current time.)
A constructor that constructs a Time object with a specified elapsed time
since midnight, January 1, 1970, in milliseconds. (The values of the data
fields will represent this time.)
A constructor that constructs a Time object with the specified hour, minute,
and second.
Three getter methods for the data fields hour, minute, and second,
respectively.
A method named setTime(long elapseTime) that sets a new time
for the object using the elapsed time. For example, if the elapsed time is
555550000 milliseconds, the hour is 10, the minute is 19, and the second is
10.
Draw the UML diagram for the class and then implement the class. Write
a test program that creates two Time objects (using new Time() and new
Time(555550000)) and displays their hour, minute, and second in the format
hour:minute:second.
(Hint: The first two constructors will extract the hour, minute, and second
from the elapsed time. For the no-arg constructor, the current time can be
obtained using System.currentTimeMillis(), as shown in Listing 2.7,
ShowCurrentTime.java.)
400 Chapter 10
Object-Oriented Thinking
10.2 (The BMI class) Add the following new constructor in the BMI class:
/** Construct a BMI with the specified name, age, weight,
* feet, and inches
*/
public BMI(String name, int age, double weight, double feet,
double inches)
10.3 (The MyInteger class) Design a class named MyInteger. The class contains:
■
■
■
■
■
■
■
■
■
An int data field named value that stores the int value represented by this
object.
A constructor that creates a MyInteger object for the specified int value.
A getter method that returns the int value.
The methods isEven(), isOdd(), and isPrime() that return true if the
value in this object is even, odd, or prime, respectively.
The static methods isEven(int), isOdd(int), and isPrime(int) that
return true if the specified value is even, odd, or prime, respectively.
The static methods isEven(MyInteger), isOdd(MyInteger), and
isPrime(MyInteger) that return true if the specified value is even, odd,
or prime, respectively.
The methods equals(int) and equals(MyInteger) that return true if
the value in this object is equal to the specified value.
A static method parseInt(char[]) that converts an array of numeric
characters to an int value.
A static method parseInt(String) that converts a string into an int
value.
Draw the UML diagram for the class and then implement the class. Write a client program that tests all methods in the class.
10.4 (The MyPoint class) Design a class named MyPoint to represent a point with
x- and y-coordinates. The class contains:
■
■
■
■
■
VideoNote
The MyPoint class
The data fields x and y that represent the coordinates with getter
methods.
A no-arg constructor that creates a point (0, 0).
A constructor that constructs a point with specified coordinates.
A method named distance that returns the distance from this point to a
specified point of the MyPoint type.
A method named distance that returns the distance from this point to
another point with specified x- and y-coordinates.
Draw the UML diagram for the class and then implement the class. Write a
test program that creates the two points (0, 0) and (10, 30.5) and displays the
distance between them.
Sections 10.4–10.8
*10.5 (Displaying the prime factors) Write a program that prompts the user to enter
a positive integer and displays all its smallest factors in decreasing order. For
example, if the integer is 120, the smallest factors are displayed as 5, 3, 2, 2,
2. Use the StackOfIntegers class to store the factors (e.g., 2, 2, 2, 3, 5) and
retrieve and display them in reverse order.
*10.6 (Displaying the prime numbers) Write a program that displays all the prime
numbers less than 120 in decreasing order. Use the StackOfIntegers class
to store the prime numbers (e.g., 2, 3, 5, ...) and retrieve and display them in
reverse order.
Programming Exercises 401
**10.7 (Game: ATM machine) Use the Account class created in Programming Exercise 9.7 to simulate an ATM machine. Create ten accounts in an array with id
0, 1, . . . , 9, and initial balance $100. The system prompts the user to enter an
id. If the id is entered incorrectly, ask the user to enter a correct id. Once an id
is accepted, the main menu is displayed as shown in the sample run. You can
enter a choice 1 for viewing the current balance, 2 for withdrawing money, 3 for
depositing money, and 4 for exiting the main menu. Once you exit, the system
will prompt for an id again. Thus, once the system starts, it will not stop.
Enter an id: 4
Main menu
1: check balance
2: withdraw
3: deposit
4: exit
Enter a choice: 1
The balance is 100.0
Main menu
1: check balance
2: withdraw
3: deposit
4: exit
Enter a choice: 2
Enter an amount to withdraw: 3
Main menu
1: check balance
2: withdraw
3: deposit
4: exit
Enter a choice: 1
The balance is 97.0
Main menu
1: check balance
2: withdraw
3: deposit
4: exit
Enter a choice: 3
Enter an amount to deposit: 10
Main menu
1: check balance
2: withdraw
3: deposit
4: exit
Enter a choice: 1
The balance is 107.0
Main menu
1: check balance
2: withdraw
3: deposit
4: exit
Enter a choice: 4
Enter an id:
402 Chapter 10
Object-Oriented Thinking
***10.8 (Financial: the
Tax class) Programming Exercise 8.12 writes a program for
computing taxes using arrays. Design a class named Tax to contain the following instance data fields:
■ int filingStatus: One of the four tax-filing statuses: 0—single filer, 1—
■
■
■
married filing jointly or qualifying widow(er), 2—married filing separately,
and 3—head of household. Use the public static constants SINGLE_FILER
(0), MARRIED_JOINTLY_OR_QUALIFYING_WIDOW(ER) (1), MARRIED_
SEPARATELY (2), HEAD_OF_HOUSEHOLD (3) to represent the statuses.
int[][] brackets: Stores the tax brackets for each filing status.
double[] rates: Stores the tax rates for each bracket.
double taxableIncome: Stores the taxable income.
Provide the getter and setter methods for each data field and the getTax()
method that returns the tax. Also provide a no-arg constructor and the constructor Tax(filingStatus, brackets, rates, taxableIncome).
Draw the UML diagram for the class and then implement the class. Write a test
program that uses the Tax class to print the 2001 and 2009 tax tables for taxable
income from $50,000 to $60,000 with intervals of $1,000 for all four statuses.
The tax rates for the year 2009 were given in Table 3.2. The tax rates for 2001
are shown in Table 10.1.
TABLE 10.1 2001 United States Federal Personal Tax Rates
Tax rate
Single filers
Married filing jointly
Married filing
or qualifying widow(er) separately
Head of household
15%
Up to $27,050
Up to $45,200
Up to $22,600
Up to $36,250
27.5%
$27,051–$65,550
$45,201–$109,250
$22,601–$54,625
$36,251–$93,650
30.5%
$65,551–$136,750
$109,251–$166,500
$54,626–$83,250
$93,651–$151,650
35.5%
$136,751–$297,350
$166,501–$297,350
$83,251–$148,675
$151,651–$297,350
39.1%
$297,351 or more
$297,351 or more
$ 148,676 or more
$297,351 or more
**10.9 (The Course class) Revise the Course class as follows:
■
■
■
The array size is fixed in Listing 10.6. Improve it to automatically increase
the array size by creating a new larger array and copying the contents of the
current array to it.
Implement the dropStudent method.
Add a new method named clear() that removes all students from the
course.
Write a test program that creates a course, adds three students, removes one,
and displays the students in the course.
*10.10 (The Queue class) Section 10.6 gives a class for Stack. Design a class named
Queue for storing integers. Like a stack, a queue holds elements. In a stack, the
elements are retrieved in a last-in first-out fashion. In a queue, the elements are
retrieved in a first-in first-out fashion. The class contains:
■
■
■
■
An int[] data field named elements that stores the int values in the
queue.
A data field named size that stores the number of elements in the queue.
A constructor that creates a Queue object with default capacity 8.
The method enqueue(int v) that adds v into the queue.
Programming Exercises 403
■
■
■
The method dequeue() that removes and returns the element from the
queue.
The method empty() that returns true if the queue is empty.
The method getSize() that returns the size of the queue.
Draw an UML diagram for the class. Implement the class with the initial array
size set to 8. The array size will be doubled once the number of the elements
exceeds the size. After an element is removed from the beginning of the array,
you need to shift all elements in the array one position the left. Write a test
program that adds 20 numbers from 1 to 20 into the queue and removes these
numbers and displays them.
*10.11 (Geometry: the Circle2D class) Define the Circle2D class that contains:
■
■
■
■
■
■
■
■
■
Two double data fields named x and y that specify the center of the circle
with getter methods.
A data field radius with a getter method.
A no-arg constructor that creates a default circle with (0, 0) for (x, y) and 1
for radius.
A constructor that creates a circle with the specified x, y, and radius.
A method getArea() that returns the area of the circle.
A method getPerimeter() that returns the perimeter of the circle.
A method contains(double x, double y) that returns true if the
specified point (x, y) is inside this circle (see Figure 10.21a).
A method contains(Circle2D circle) that returns true if the specified circle is inside this circle (see Figure 10.21b).
A method overlaps(Circle2D circle) that returns true if the specified circle overlaps with this circle (see Figure 10.21c).
p
(a)
(b)
(c)
FIGURE 10.21 (a) A point is inside the circle. (b) A circle is inside another circle. (c) A circle
overlaps another circle.
Draw the UML diagram for the class and then implement the class. Write a test
program that creates a Circle2D object c1 (new Circle2D(2, 2, 5.5)),
displays its area and perimeter, and displays the result of c1.contains(3,
3), c1.contains(new Circle2D(4, 5, 10.5)), and c1.overlaps(new
Circle2D(3, 5, 2.3)).
***10.12 (Geometry: the Triangle2D class) Define the Triangle2D class that contains:
■
■
■
■
■
■
Three points named p1, p2, and p3 of the type MyPoint with getter and
setter methods. MyPoint is defined in Programming Exercise 10.4.
A no-arg constructor that creates a default triangle with the points (0, 0), (1,
1), and (2, 5).
A constructor that creates a triangle with the specified points.
A method getArea() that returns the area of the triangle.
A method getPerimeter() that returns the perimeter of the triangle.
A method contains(MyPoint p) that returns true if the specified point
p is inside this triangle (see Figure 10.22a).
404 Chapter 10
Object-Oriented Thinking
■
■
A method contains(Triangle2D t) that returns true if the specified
triangle is inside this triangle (see Figure 10.22b).
A method overlaps(Triangle2D t) that returns true if the specified
triangle overlaps with this triangle (see Figure 10.22c).
p
(a)
(b)
(c)
FIGURE 10.22 (a) A point is inside the triangle. (b) A triangle is inside another triangle.
(c) A triangle overlaps another triangle.
Draw the UML diagram for the class and then implement the class. Write
a test program that creates a Triangle2D objects t1 using the constructor
new Triangle2D(new MyPoint(2.5, 2), new MyPoint(4.2, 3),
new MyPoint(5, 3.5)), displays its area and perimeter, and displays the
result of t1.contains(3, 3), r1.contains(new Triangle2D(new
MyPoint(2.9, 2), new MyPoint(4, 1), MyPoint(1, 3.4))), and t1.
overlaps(new Triangle2D(new MyPoint(2, 5.5), new MyPoint(4,
-3), MyPoint(2, 6.5))).
(Hint: For the formula to compute the area of a triangle, see Programming Exercise 2.19. To detect whether a point is inside a triangle, draw three dashed lines,
as shown in Figure 10.23. If the point is inside a triangle, each dashed line
should intersect a side only once. If a dashed line intersects a side twice, then
the point must be outside the triangle. For the algorithm of finding the intersecting point of two lines, see Programming Exercise 3.25.)
p
p
(a)
(b)
FIGURE 10.23 (a) A point is inside the triangle. (b) A point is outside the triangle.
*10.13 (Geometry: the MyRectangle2D class) Define the MyRectangle2D class that
contains:
■
■
■
■
Two double data fields named x and y that specify the center of the rectangle with getter and setter methods. (Assume that the rectangle sides are
parallel to x- or y- axes.)
The data fields width and height with getter and setter methods.
A no-arg constructor that creates a default rectangle with (0, 0) for (x, y) and
1 for both width and height.
A constructor that creates a rectangle with the specified x, y, width, and
height.
Programming Exercises 405
■
■
■
■
■
A method getArea() that returns the area of the rectangle.
A method getPerimeter() that returns the perimeter of the rectangle.
A method contains(double x, double y) that returns true if the
specified point (x, y) is inside this rectangle (see Figure 10.24a).
A method contains(MyRectangle2D r) that returns true if the specified rectangle is inside this rectangle (see Figure 10.24b).
A method overlaps(MyRectangle2D r) that returns true if the specified rectangle overlaps with this rectangle (see Figure 10.24c).
p
(a)
(b)
(c)
(d)
FIGURE 10.24 A point is inside the rectangle. (b) A rectangle is inside another rectangle.
(c) A rectangle overlaps another rectangle. (d) Points are enclosed inside a rectangle.
Draw the UML diagram for the class and then implement the class. Write a test
program that creates a MyRectangle2D object r1 (new MyRectangle2D(2,
2, 5.5, 4.9)), displays its area and perimeter, and displays the result of
r1.contains(3, 3), r1.contains(new MyRectangle2D(4, 5, 10.5,
3.2)), and r1.overlaps(new MyRectangle2D(3, 5, 2.3, 5.4)).
*10.14 (The MyDate class) Design a class named MyDate. The class contains:
■
■
■
■
■
■
The data fields year, month, and day that represent a date. month is
0-based, i.e., 0 is for January.
A no-arg constructor that creates a MyDate object for the current date.
A constructor that constructs a MyDate object with a specified elapsed time
since midnight, January 1, 1970, in milliseconds.
A constructor that constructs a MyDate object with the specified year,
month, and day.
Three getter methods for the data fields year, month, and day, respectively.
A method named setDate(long elapsedTime) that sets a new date for
the object using the elapsed time.
Draw the UML diagram for the class and then implement the class. Write a
test program that creates two MyDate objects (using new MyDate() and new
MyDate(34355555133101L)) and displays their year, month, and day.
(Hint: The first two constructors will extract the year, month, and day from
the elapsed time. For example, if the elapsed time is 561555550000 milliseconds, the year is 1987, the month is 9, and the day is 18. You may use the
GregorianCalendar class discussed in Programming Exercise 9.5 to simplify coding.)
*10.15 (Geometry: the bounding rectangle) A bounding rectangle is the minimum rectangle that encloses a set of points in a two-dimensional plane, as shown in
Figure 10.24d. Write a method that returns a bounding rectangle for a set of
points in a two-dimensional plane, as follows:
public static MyRectangle2D getRectangle(double[][] points)
406 Chapter 10
Object-Oriented Thinking
The Rectangle2D class is defined in Programming Exercise 10.13. Write a
test program that prompts the user to enter five points and displays the bounding
rectangle’s center, width, and height. Here is a sample run:
Enter five points: 1.0 2.5 3 4 5 6 7 8 9 10
The bounding rectangle's center (5.0, 6.25), width 8.0, height 7.5
Section 10.9
*10.16 (Divisible by 2 or 3) Find the first ten numbers with 50 decimal digits that are
divisible by 2 or 3.
*10.17 (Square numbers) Find the first ten square numbers that are greater than
Long.MAX_VALUE. A square number is a number in the form of n2. For example, 4, 9, and 16 are square numbers. Find an efficient approach to run your
program fast.
*10.18 (Large prime numbers) Write a program that finds five prime numbers larger
than Long.MAX_VALUE.
*10.19 (Mersenne prime) A prime number is called a Mersenne prime if it can be written in the form 2p - 1 for some positive integer p. Write a program that finds
all Mersenne primes with p … 100 and displays the output as shown below.
(Hint: You have to use BigInteger to store the number, because it is too big to
be stored in long. Your program may take several hours to run.)
p
2^p – 1
2
3
5
3
7
31
...
*10.20 (Approximate e) Programming Exercise 5.26 approximates e using the following
series:
e = 1 +
1
1
1
1
1
+
+
+
+ c +
1!
2!
3!
4!
i!
In order to get better precision, use BigDecimal with 25 digits of precision in
the computation. Write a program that displays the e value for i = 100, 200, . . . ,
and 1000.
10.21 (Divisible by 5 or 6) Find the first ten numbers greater than Long.MAX_VALUE
that are divisible by 5 or 6.
Sections 10.10–10.11
**10.22 (Implement the String class) The String class is provided in the Java library.
Provide your own implementation for the following methods (name the new
class MyString1):
public
public
public
public
public
public
public
MyString1(char[] chars);
char charAt(int index);
int length();
MyString1 substring(int begin, int end);
MyString1 toLowerCase();
boolean equals(MyString1 s);
static MyString1 valueOf(int i);
Programming Exercises 407
**10.23 (Implement the String class) The String class is provided in the Java library.
Provide your own implementation for the following methods (name the new
class MyString2):
public
public
public
public
public
public
MyString2(String s);
int compare(String s);
MyString2 substring(int begin);
MyString2 toUpperCase();
char[] toChars();
static MyString2 valueOf(boolean b);
10.24 (Implement the Character class) The Character class is provided in the Java
library. Provide your own implementation for this class. Name the new class
MyCharacter.
**10.25 (New string split method) The split method in the String class returns an
array of strings consisting of the substrings split by the delimiters. However, the
delimiters are not returned. Implement the following new method that returns
an array of strings consisting of the substrings split by the matching delimiters,
including the matching delimiters.
public static String[] split(String s, String regex)
*10.26
For example, split("ab#12#453", "#") returns ab, #, 12, #, 453 in an
array of String, and split("a?b?gf#e", "[?#]") returns a, b, ?, b, gf,
#, and e in an array of String.
(Calculator) Revise Listing 7.9, Calculator.java, to accept an expression as
a string in which the operands and operator are separated by zero or more
spaces. For example, 3+4 and 3 + 4 are acceptable expressions. Here is a
sample run:
**10.27 (Implement the StringBuilder class) The StringBuilder class is provided
in the Java library. Provide your own implementation for the following methods
(name the new class MyStringBuilder1):
public
public
public
public
public
public
public
public
MyStringBuilder1(String s);
MyStringBuilder1 append(MyStringBuilder1 s);
MyStringBuilder1 append(int i);
int length();
char charAt(int index);
MyStringBuilder1 toLowerCase();
MyStringBuilder1 substring(int begin, int end);
String toString();
408 Chapter 10
Object-Oriented Thinking
**10.28 (Implement the StringBuilder class) The StringBuilder class is provided
in the Java library. Provide your own implementation for the following methods
(name the new class MyStringBuilder2):
public
public
public
public
public
public
public
MyStringBuilder2();
MyStringBuilder2(char[] chars);
MyStringBuilder2(String s);
MyStringBuilder2 insert(int offset, MyStringBuilder2 s);
MyStringBuilder2 reverse();
MyStringBuilder2 substring(int begin);
MyStringBuilder2 toUpperCase();
CHAPTER
11
INHERITANCE
AND POLYMORPHISM
Objectives
■
To define a subclass from a superclass through inheritance (§11.2).
■
To invoke the superclass’s constructors and methods using the super
keyword (§11.3).
■
To override instance methods in the subclass (§11.4).
■
To distinguish differences between overriding and overloading
(§11.5).
■
To explore the toString() method in the Object class (§11.6).
■
To discover polymorphism and dynamic binding (§§11.7–11.8).
■
To describe casting and explain why explicit downcasting is necessary
(§11.9).
■
To explore the equals method in the Object class (§11.10).
■
To store, retrieve, and manipulate objects in an ArrayList (§11.11).
■
To construct an array list from an array, to sort and shuffle a list, and
to obtain max and min element from a list (§11.12).
■
To implement a Stack class using ArrayList (§11.13).
■
To enable data and methods in a superclass accessible from subclasses
using the protected visibility modifier (§11.14).
■
To prevent class extending and method overriding using the final
modifier (§11.15).
410 Chapter 11
Inheritance and Polymorphism
11.1 Introduction
Key
Point
Object-oriented programming allows you to define new classes from existing classes.
This is called inheritance.
As discussed earlier in the book, the procedural paradigm focuses on designing methods and
the object-oriented paradigm couples data and methods together into objects. Software design
using the object-oriented paradigm focuses on objects and operations on objects. The objectoriented approach combines the power of the procedural paradigm with an added dimension
that integrates data with operations into objects.
Inheritance is an important and powerful feature for reusing software. Suppose you need
to define classes to model circles, rectangles, and triangles. These classes have many common
features. What is the best way to design these classes so as to avoid redundancy and make the
system easy to comprehend and easy to maintain? The answer is to use inheritance.
inheritance
why inheritance?
11.2 Superclasses and Subclasses
Key
Point
VideoNote
Geometric class hierarchy
subclass
superclass
Inheritance enables you to define a general class (i.e., a superclass) and later extend it
to more specialized classes (i.e., subclasses).
You use a class to model objects of the same type. Different classes may have some common properties and behaviors, which can be generalized in a class that can be shared by other
classes. You can define a specialized class that extends the generalized class. The specialized
classes inherit the properties and methods from the general class.
Consider geometric objects. Suppose you want to design the classes to model geometric
objects such as circles and rectangles. Geometric objects have many common properties and
behaviors. They can be drawn in a certain color and be filled or unfilled. Thus a general class
GeometricObject can be used to model all geometric objects. This class contains the properties color and filled and their appropriate getter and setter methods. Assume that this class
also contains the dateCreated property and the getDateCreated() and toString()
methods. The toString() method returns a string representation of the object. Since a circle
is a special type of geometric object, it shares common properties and methods with other
geometric objects. Thus it makes sense to define the Circle class that extends the GeometricObject class. Likewise, Rectangle can also be defined as a subclass of GeometricObject. Figure 11.1 shows the relationship among these classes. A triangular arrow pointing to
the superclass is used to denote the inheritance relationship between the two classes involved.
In Java terminology, a class C1 extended from another class C2 is called a subclass, and C2
is called a superclass. A superclass is also referred to as a parent class or a base class, and a
subclass as a child class, an extended class, or a derived class. A subclass inherits accessible
data fields and methods from its superclass and may also add new data fields and methods.
The Circle class inherits all accessible data fields and methods from the GeometricObject
class. In addition, it has a new data field, radius, and its associated getter and setter methods.
The Circle class also contains the getArea(), getPerimeter(), and getDiameter()
methods for returning the area, perimeter, and diameter of the circle.
The Rectangle class inherits all accessible data fields and methods from the GeometricObject class. In addition, it has the data fields width and height and their associated
getter and setter methods. It also contains the getArea() and getPerimeter() methods for
returning the area and perimeter of the rectangle.
The GeometricObject, Circle, and Rectangle classes are shown in Listings 11.1,
11.2, and 11.3.
Note
avoid naming conflicts
To avoid a naming conflict with the improved GeometricObject, Circle,
and Rectangle classes introduced in Chapter 13, we’ll name these classes
11.2 Superclasses and Subclasses 411
GeometricObject
-color: String
The color of the object (default: white).
-filled: boolean
Indicates whether the object is filled with a color (default: false).
-dateCreated: java.util.Date
The date when the object was created.
+GeometricObject()
Creates a GeometricObject.
+GeometricObject(color: String,
filled: boolean)
+getColor(): String
+setColor(color: String): void
Creates a GeometricObject with the specified color and filled
values.
Returns the color.
+isFilled(): boolean
+setFilled(filled: boolean): void
Returns the filled property.
+getDateCreated(): java.util.Date
Returns the dateCreated.
+toString(): String
Returns a string representation of this object.
Sets a new color.
Sets a new filled property.
Circle
-radius: double
Rectangle
-width: double
-height: double
+Circle()
+Circle(radius: double)
+Circle(radius: double, color: String,
filled: boolean)
+Rectangle()
+Rectangle(width: double, height: double)
+setRadius(radius: double): void
+Rectangle(width: double, height: double
color: String, filled: boolean)
+getWidth(): double
+getArea(): double
+setWidth(width: double): void
+getPerimeter(): double
+getHeight(): double
+getDiameter(): double
+setHeight(height: double): void
+printCircle(): void
+getArea(): double
+getRadius(): double
+getPerimeter(): double
FIGURE 11.1 The GeometricObject class is the superclass for Circle and Rectangle.
SimpleGeometricObject, CircleFromSimpleGeometricObject, and RectangleFromSimpleGeometricObject in this chapter. For simplicity, we will still
refer to them in the text as GeometricObject, Circle, and Rectangle classes.
The best way to avoid naming conflicts is to place these classes in different packages.
However, for simplicity and consistency, all classes in this book are placed in the default
package.
LISTING 11.1
1
2
3
4
5
6
7
8
9
SimpleGeometricObject.java
public class SimpleGeometricObject {
private String color = "white";
private boolean filled;
private java.util.Date dateCreated;
/** Construct a default geometric object */
public SimpleGeometricObject() {
dateCreated = new java.util.Date();
}
data fields
constructor
date constructed
412 Chapter 11
Inheritance and Polymorphism
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
/** Construct a geometric object with the specified color
* and filled value */
public SimpleGeometricObject(String color, boolean filled) {
dateCreated = new java.util.Date();
this.color = color;
this.filled = filled;
}
/** Return color */
public String getColor() {
return color;
}
/** Set a new color */
public void setColor(String color) {
this.color = color;
}
/** Return filled. Since filled is boolean,
its getter method is named isFilled */
public boolean isFilled() {
return filled;
}
/** Set a new filled */
public void setFilled(boolean filled) {
this.filled = filled;
}
/** Get dateCreated */
public java.util.Date getDateCreated() {
return dateCreated;
}
/** Return a string representation of this object */
public String toString() {
return "created on " + dateCreated + "\ncolor: " + color +
" and filled: " + filled;
}
}
LISTING 11.2 CircleFromSimpleGeometricObject.java
extends superclass
data fields
constructor
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
public class CircleFromSimpleGeometricObject
extends SimpleGeometricObject
private double radius;
public CircleFromSimpleGeometricObject() {
}
public CircleFromSimpleGeometricObject(double radius) {
this.radius = radius;
}
public CircleFromSimpleGeometricObject(double radius,
String color, boolean filled) {
this.radius = radius;
setColor(color);
setFilled(filled);
11.2 Superclasses and Subclasses 413
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
}
/** Return radius */
public double getRadius() {
return radius;
}
methods
/** Set a new radius */
public void setRadius(double radius) {
this.radius = radius;
}
/** Return area */
public double getArea() {
return radius * radius * Math.PI;
}
/** Return diameter */
public double getDiameter() {
return 2 * radius;
}
/** Return perimeter */
public double getPerimeter() {
return 2 * radius * Math.PI;
}
/** Print the circle info
public void printCircle()
System.out.println("The
" and the radius is "
}
*/
{
circle is created " + getDateCreated() +
+ radius);
}
The Circle class (Listing 11.2) extends the GeometricObject class (Listing 11.1) using
the following syntax:
Subclass
Superclass
public class Circle extends GeometricObject
The keyword extends (lines 1–2) tells the compiler that the Circle class extends the
GeometricObject class, thus inheriting the methods getColor, setColor, isFilled,
setFilled, and toString.
The overloaded constructor Circle(double radius, String color, boolean
filled) is implemented by invoking the setColor and setFilled methods to set the color
and filled properties (lines 12–17). These two public methods are defined in the superclass
GeometricObject and are inherited in Circle, so they can be used in the Circle class.
You might attempt to use the data fields color and filled directly in the constructor as
follows:
public CircleFromSimpleGeometricObject(
double radius, String color, boolean filled) {
this.radius = radius;
this.color = color; // Illegal
this.filled = filled; // Illegal
}
private member in superclass
414 Chapter 11
Inheritance and Polymorphism
This is wrong, because the private data fields color and filled in the GeometricObject
class cannot be accessed in any class other than in the GeometricObject class itself. The
only way to read and modify color and filled is through their getter and setter methods.
The Rectangle class (Listing 11.3) extends the GeometricObject class (Listing 11.1)
using the following syntax:
Subclass
Superclass
public class Rectangle extends GeometricObject
The keyword extends (lines 1–2) tells the compiler that the Rectangle class extends the
GeometricObject class, thus inheriting the methods getColor, setColor, isFilled,
setFilled, and toString.
LISTING 11.3 RectangleFromSimpleGeometricObject.java
extends superclass
data fields
constructor
methods
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
public class RectangleFromSimpleGeometricObject
extends SimpleGeometricObject {
private double width;
private double height;
public RectangleFromSimpleGeometricObject() {
}
public RectangleFromSimpleGeometricObject(
double width, double height) {
this.width = width;
this.height = height;
}
public RectangleFromSimpleGeometricObject(
double width, double height, String color, boolean filled) {
this.width = width;
this.height = height;
setColor(color);
setFilled(filled);
}
/** Return width */
public double getWidth() {
return width;
}
/** Set a new width */
public void setWidth(double width) {
this.width = width;
}
/** Return height */
public double getHeight() {
return height;
}
/** Set a new height */
public void setHeight(double height) {
this.height = height;
}
11.2 Superclasses and Subclasses 415
42
43
44
45
46
47
48
49
50
51
52
/** Return area */
public double getArea() {
return width * height;
}
/** Return perimeter */
public double getPerimeter() {
return 2 * (width + height);
}
}
The code in Listing 11.4 creates objects of Circle and Rectangle and invokes the methods
on these objects. The toString() method is inherited from the GeometricObject class
and is invoked from a Circle object (line 5) and a Rectangle object (line 13).
LISTING 11.4
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
TestCircleRectangle.java
public class TestCircleRectangle {
public static void main(String[] args) {
CircleFromSimpleGeometricObject circle =
new CircleFromSimpleGeometricObject(1);
System.out.println("A circle " + circle.toString());
System.out.println("The color is " + circle.getColor());
System.out.println("The radius is " + circle.getRadius());
System.out.println("The area is " + circle.getArea());
System.out.println("The diameter is " + circle.getDiameter());
RectangleFromSimpleGeometricObject rectangle =
new RectangleFromSimpleGeometricObject(2, 4);
System.out.println("\nA rectangle " + rectangle.toString());
System.out.println("The area is " + rectangle.getArea());
System.out.println("The perimeter is " +
rectangle.getPerimeter());
Circle object
invoke toString
invoke getColor
Rectangle object
invoke toString
}
}
A circle created on Thu Feb 10 19:54:25 EST 2011
color: white and filled: false
The color is white
The radius is 1.0
The area is 3.141592653589793
The diameter is 2.0
A rectangle created on Thu Feb 10 19:54:25 EST 2011
color: white and filled: false
The area is 8.0
The perimeter is 12.0
Note the following points regarding inheritance:
■
Contrary to the conventional interpretation, a subclass is not a subset of its superclass.
In fact, a subclass usually contains more information and methods than its superclass.
more in subclass
■
Private data fields in a superclass are not accessible outside the class. Therefore,
they cannot be used directly in a subclass. They can, however, be accessed/mutated
through public accessors/mutators if defined in the superclass.
private data fields
416 Chapter 11
Inheritance and Polymorphism
nonextensible is-a
■
Not all is-a relationships should be modeled using inheritance. For example, a square
is a rectangle, but you should not extend a Square class from a Rectangle class,
because the width and height properties are not appropriate for a square. Instead,
you should define a Square class to extend the GeometricObject class and define
the side property for the side of a square.
no blind extension
■
Inheritance is used to model the is-a relationship. Do not blindly extend a class just
for the sake of reusing methods. For example, it makes no sense for a Tree class to
extend a Person class, even though they share common properties such as height
and weight. A subclass and its superclass must have the is-a relationship.
■
Some programming languages allow you to derive a subclass from several classes.
This capability is known as multiple inheritance. Java, however, does not allow multiple inheritance. A Java class may inherit directly from only one superclass. This
restriction is known as single inheritance. If you use the extends keyword to define
a subclass, it allows only one parent class. Nevertheless, multiple inheritance can be
achieved through interfaces, which will be introduced in Section 13.4.
multiple inheritance
single inheritance
✓
Check
Point
11.1 True or false? A subclass is a subset of a superclass.
11.2 What keyword do you use to define a subclass?
11.3 What is single inheritance? What is multiple inheritance? Does Java support multiple
inheritance?
11.3 Using the super Keyword
Key
Point
The keyword super refers to the superclass and can be used to invoke the superclass’s methods and constructors.
A subclass inherits accessible data fields and methods from its superclass. Does it inherit
constructors? Can the superclass’s constructors be invoked from a subclass? This section
addresses these questions and their ramifications.
Section 9.14, The this Reference, introduced the use of the keyword this to reference
the calling object. The keyword super refers to the superclass of the class in which super
appears. It can be used in two ways:
■
To call a superclass constructor.
■
To call a superclass method.
11.3.1
Calling Superclass Constructors
A constructor is used to construct an instance of a class. Unlike properties and methods, the
constructors of a superclass are not inherited by a subclass. They can only be invoked from
the constructors of the subclasses using the keyword super.
The syntax to call a superclass’s constructor is:
super(), or super(parameters);
The statement super() invokes the no-arg constructor of its superclass, and the statement
super(arguments) invokes the superclass constructor that matches the arguments. The
statement super() or super(arguments) must be the first statement of the subclass’s constructor; this is the only way to explicitly invoke a superclass constructor. For example, the
constructor in lines 12–17 in Listing 11.2 can be replaced by the following code:
public CircleFromSimpleGeometricObject(
double radius, String color, boolean filled) {
11.3 Using the super Keyword 417
super(color, filled);
this.radius = radius;
}
Caution
You must use the keyword super to call the superclass constructor, and the call must
be the first statement in the constructor. Invoking a superclass constructor’s name in a
subclass causes a syntax error.
11.3.2
Constructor Chaining
A constructor may invoke an overloaded constructor or its superclass constructor. If neither
is invoked explicitly, the compiler automatically puts super() as the first statement in the
constructor. For example:
public ClassName() {
// some statements
}
Equivalent
public ClassName() {
super();
// some statements
}
public ClassName(double d) {
// some statements
}
Equivalent
public ClassName(double d) {
super();
// some statements
}
In any case, constructing an instance of a class invokes the constructors of all the superclasses
along the inheritance chain. When constructing an object of a subclass, the subclass constructor first invokes its superclass constructor before performing its own tasks. If the superclass
is derived from another class, the superclass constructor invokes its parent-class constructor
before performing its own tasks. This process continues until the last constructor along the
inheritance hierarchy is called. This is called constructor chaining.
Consider the following code:
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
constructor chaining
public class Faculty extends Employee {
public static void main(String[] args) {
new Faculty();
}
public Faculty() {
System.out.println("(4) Performs Faculty's tasks");
}
}
class Employee extends Person {
public Employee() {
this("(2) Invoke Employee's overloaded constructor");
System.out.println("(3) Performs Employee's tasks ");
}
public Employee(String s) {
System.out.println(s);
}
}
class Person {
invoke overloaded
constructor
418 Chapter 11
Inheritance and Polymorphism
23
24
25
26
(1)
(2)
(3)
(4)
public Person() {
System.out.println("(1) Performs Person's tasks");
}
}
Performs Person's tasks
Invoke Employee's overloaded constructor
Performs Employee's tasks
Performs Faculty's tasks
The program produces the preceding output. Why? Let us discuss the reason. In line 3,
new Faculty() invokes Faculty’s no-arg constructor. Since Faculty is a subclass of
Employee, Employee’s no-arg constructor is invoked before any statements in Faculty’s
constructor are executed. Employee’s no-arg constructor invokes Employee’s second constructor (line 13). Since Employee is a subclass of Person, Person’s no-arg constructor is
invoked before any statements in Employee’s second constructor are executed. This process
is illustrated in the following figure.
Faculty() {
Employee() {
this("(2) ...");
Performs Faculty's
tasks;
}
Employee(String s) {
Performs Employee's
tasks;
}
Person() {
Performs Employee's
tasks;
}
Performs Person's
tasks;
}
Caution
If a class is designed to be extended, it is better to provide a no-arg constructor to avoid
programming errors. Consider the following code:
no-arg constructor
1
2
3
4
5
6
7
8
public class Apple extends Fruit {
}
class Fruit {
public Fruit(String name) {
System.out.println("Fruit's constructor is invoked");
}
}
Since no constructor is explicitly defined in Apple, Apple’s default no-arg constructor
is defined implicitly. Since Apple is a subclass of Fruit, Apple’s default constructor
automatically invokes Fruit’s no-arg constructor. However, Fruit does not have a
no-arg constructor, because Fruit has an explicit constructor defined. Therefore, the
program cannot be compiled.
Design Guide
If possible, you should provide a no-arg constructor for every class to make the class
easy to extend and to avoid errors.
no-arg constructor
11.3.3
Calling Superclass Methods
The keyword super can also be used to reference a method other than the constructor in the
superclass. The syntax is:
super.method(parameters);
11.4 Overriding Methods 419
You could rewrite the printCircle() method in the Circle class as follows:
public void printCircle() {
System.out.println("The circle is created " +
super.getDateCreated() + " and the radius is " + radius);
}
It is not necessary to put super before getDateCreated() in this case, however, because
getDateCreated is a method in the GeometricObject class and is inherited by the Circle
class. Nevertheless, in some cases, as shown in the next section, the keyword super is needed.
11.4 What is the output of running the class C in (a)? What problem arises in compiling the
program in (b)?
class A {
public A() {
System.out.println(
"A's no-arg constructor is invoked");
}
}
✓
Check
Point
class A {
public A(int x) {
}
}
class B extends A {
}
class B extends A {
public B() {
}
}
public class C {
public static void main(String[] args) {
B b = new B();
}
}
public class C {
public static void main(String[] args) {
B b = new B();
}
}
(a)
(b)
11.5 How does a subclass invoke its superclass’s constructor?
11.6 True or false? When invoking a constructor from a subclass, its superclass’s no-arg
constructor is always invoked.
11.4 Overriding Methods
To override a method, the method must be defined in the subclass using the same signature and the same return type as in its superclass.
A subclass inherits methods from a superclass. Sometimes it is necessary for the subclass to modify
the implementation of a method defined in the superclass. This is referred to as method overriding.
The toString method in the GeometricObject class (lines 46–49 in Listing 11.1)
returns the string representation of a geometric object. This method can be overridden to
return the string representation of a circle. To override it, add the following new method in the
Circle class in Listing 11.2.
1
2
3
4
5
6
7
8
9
Key
Point
method overriding
public class CircleFromSimpleGeometricObject
extends SimpleGeometricObject {
// Other methods are omitted
// Override the toString method defined in the superclass
public String toString() {
return super.toString() + "\nradius is " + radius;
}
}
toString in superclass
420 Chapter 11
Inheritance and Polymorphism
no super.super.methodName()
override accessible instance
method
The toString() method is defined in the GeometricObject class and modified in the Circle
class. Both methods can be used in the Circle class. To invoke the toString method defined in
the GeometricObject class from the Circle class, use super.toString() (line 7).
Can a subclass of Circle access the toString method defined in the GeometricObject class using syntax such as super.super.toString()? No. This is a syntax error.
Several points are worth noting:
■
An instance method can be overridden only if it is accessible. Thus a private method cannot be overridden, because it is not accessible outside its own class. If a method defined
in a subclass is private in its superclass, the two methods are completely unrelated.
■
Like an instance method, a static method can be inherited. However, a static method
cannot be overridden. If a static method defined in the superclass is redefined in a
subclass, the method defined in the superclass is hidden. The hidden static methods
can be invoked using the syntax SuperClassName.staticMethodName.
cannot override static method
✓
Check
Point
11.7
11.8
11.9
11.10
True or false? You can override a private method defined in a superclass.
True or false? You can override a static method defined in a superclass.
How do you explicitly invoke a superclass’s constructor from a subclass?
How do you invoke an overridden superclass method from a subclass?
11.5 Overriding vs. Overloading
Key
Point
Overloading means to define multiple methods with the same name but different signatures. Overriding means to provide a new implementation for a method in the subclass.
You learned about overloading methods in Section 6.8. To override a method, the method
must be defined in the subclass using the same signature and the same return type.
Let us use an example to show the differences between overriding and overloading. In (a)
below, the method p(double i) in class A overrides the same method defined in class B. In
(b), however, the class A has two overloaded methods: p(double i) and p(int i). The
method p(double i) is inherited from B.
public class Test {
public static void main(String[] args) {
A a = new A();
a.p(10);
a.p(10.0);
}
}
public class Test {
public static void main(String[] args) {
A a = new A();
a.p(10);
a.p(10.0);
}
}
class B {
public void p(double i) {
System.out.println(i * 2);
}
}
class B {
public void p(double i) {
System.out.println(i * 2);
}
}
class A extends B {
// This method overrides the method in B
public void p(double i) {
System.out.println(i);
}
}
class A extends B {
// This method overloads the method in B
public void p(int i) {
System.out.println(i);
}
}
(a)
(b)
11.5 Overriding vs. Overloading 421
When you run the Test class in (a), both a.p(10) and a.p(10.0) invoke the p(double
i) method defined in class A to display 10.0. When you run the Test class in (b), a.p(10)
invokes the p(int i) method defined in class A to display 10, and a.p(10.0) invokes the
p(double i) method defined in class B to display 20.0.
Note the following:
■
Overridden methods are in different classes related by inheritance; overloaded methods can be either in the same class or different classes related by inheritance.
■
Overridden methods have the same signature and return type; overloaded methods
have the same name but a different parameter list.
To avoid mistakes, you can use a special Java syntax, called override annotation, to place
@Override before the method in the subclass. For example:
1
2
3
4
5
6
7
8
9
override annotation
public class CircleFromSimpleGeometricObject
extends SimpleGeometricObject {
// Other methods are omitted
@Override
public String toString() {
return super.toString() + "\nradius is " + radius;
}
toString in superclass
}
This annotation denotes that the annotated method is required to override a method in the
superclass. If a method with this annotation does not override its superclass’s method, the
compiler will report an error. For example, if toString is mistyped as tostring, a compile
error is reported. If the override annotation isn’t used, the compile won’t report an error. Using
annotation avoids mistakes.
11.11 Identify the problems in the following code:
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
public class Circle {
private double radius;
public Circle(double radius) {
radius = radius;
}
public double getRadius() {
return radius;
}
public double getArea() {
return radius * radius * Math.PI;
}
}
class B extends Circle {
private double length;
B(double radius, double length) {
Circle(radius);
length = length;
}
@Override
✓
Check
Point
422 Chapter 11
Inheritance and Polymorphism
26
27
28
29
public double getArea() {
return getArea() * length;
}
}
11.12 Explain the difference between method overloading and method overriding.
11.13 If a method in a subclass has the same signature as a method in its superclass with the
same return type, is the method overridden or overloaded?
11.14 If a method in a subclass has the same signature as a method in its superclass with a
different return type, will this be a problem?
11.15 If a method in a subclass has the same name as a method in its superclass with different parameter types, is the method overridden or overloaded?
11.16 What is the benefit of using the @Override annotation?
11.6 The Object Class and Its toString() Method
Key
Point
Every class in Java is descended from the java.lang.Object class.
If no inheritance is specified when a class is defined, the superclass of the class is Object by
default. For example, the following two class definitions are the same:
public class
...
}
toString()
ClassName {
Equivalent
public class
...
}
ClassName
extends
Object {
Classes such as String, StringBuilder, Loan, and GeometricObject are implicitly subclasses of Object (as are all the main classes you have seen in this book so far).
It is important to be familiar with the methods provided by the Object class so that
you can use them in your classes. This section introduces the toString method in the
Object class.
The signature of the toString() method is:
public String toString()
string representation
Invoking toString() on an object returns a string that describes the object. By default, it
returns a string consisting of a class name of which the object is an instance, an at sign (@),
and the object’s memory address in hexadecimal. For example, consider the following code
for the Loan class defined in Listing 10.2:
Loan loan = new Loan();
System.out.println(loan.toString());
The output for this code displays something like Loan@15037e5. This message is not very
helpful or informative. Usually you should override the toString method so that it returns
a descriptive string representation of the object. For example, the toString method in the
Object class was overridden in the GeometricObject class in lines 46–49 in Listing 11.1
as follows:
public String toString() {
return "created on " + dateCreated + "\ncolor: " + color +
" and filled: " + filled;
}
11.7 Polymorphism 423
Note
You can also pass an object to invoke System.out.println(object) or
System.out.print(object). This is equivalent to invoking System.out
or System.out.print(object.
.println(object.toString())
toString()). Thus, you could replace System.out.println(loan
.toString()) with System.out.println(loan).
print object
11.7 Polymorphism
Polymorphism means that a variable of a supertype can refer to a subtype object.
The three pillars of object-oriented programming are encapsulation, inheritance, and polymorphism. You have already learned the first two. This section introduces polymorphism.
First, let us define two useful terms: subtype and supertype. A class defines a type. A
type defined by a subclass is called a subtype, and a type defined by its superclass is called
a supertype. Therefore, you can say that Circle is a subtype of GeometricObject and
GeometricObject is a supertype for Circle.
The inheritance relationship enables a subclass to inherit features from its superclass with
additional new features. A subclass is a specialization of its superclass; every instance of a
subclass is also an instance of its superclass, but not vice versa. For example, every circle
is a geometric object, but not every geometric object is a circle. Therefore, you can always
pass an instance of a subclass to a parameter of its superclass type. Consider the code in
Listing 11.5.
LISTING 11.5
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
Key
Point
subtype
supertype
PolymorphismDemo.java
public class PolymorphismDemo {
/** Main method */
public static void main(String[] args) {
// Display circle and rectangle properties
displayObject(new CircleFromSimpleGeometricObject
(1, "red", false));
displayObject(new RectangleFromSimpleGeometricObject
(1, 1, "black", true));
}
polymorphic call
polymorphic call
/** Display geometric object properties */
public static void displayObject(SimpleGeometricObject object) {
System.out.println("Created on " + object.getDateCreated() +
". Color is " + object.getColor());
}
}
Created on Mon Mar 09 19:25:20 EDT 2011. Color is red
Created on Mon Mar 09 19:25:20 EDT 2011. Color is black
The method displayObject (line 12) takes a parameter of the GeometricObject type.
You can invoke displayObject by passing any instance of GeometricObject (e.g., new
CircleFromSimpleGeometricObject(1, "red", false) and new RectangleFromSimpleGeometricObject(1, 1, "black", false) in lines 5–8). An object of
a subclass can be used wherever its superclass object is used. This is commonly known as
polymorphism (from a Greek word meaning “many forms”). In simple terms, polymorphism
means that a variable of a supertype can refer to a subtype object.
what is polymorphism?
424 Chapter 11
Inheritance and Polymorphism
11.8 Dynamic Binding
Key
Point
A method can be implemented in several classes along the inheritance chain. The JVM
decides which method is invoked at runtime.
A method can be defined in a superclass and overridden in its subclass. For example, the
toString() method is defined in the Object class and overridden in GeometricObject.
Consider the following code:
Object o = new GeometricObject();
System.out.println(o.toString());
declared type
actual type
dynamic binding
Which toString() method is invoked by o? To answer this question, we first introduce
two terms: declared type and actual type. A variable must be declared a type. The type that
declares a variable is called the variable’s declared type. Here o’s declared type is Object. A
variable of a reference type can hold a null value or a reference to an instance of the declared
type. The instance may be created using the constructor of the declared type or its subtype.
The actual type of the variable is the actual class for the object referenced by the variable.
Here o’s actual type is GeometricObject, because o references an object created using new
GeometricObject(). Which toString() method is invoked by o is determined by o’s
actual type. This is known as dynamic binding.
Dynamic binding works as follows: Suppose an object o is an instance of classes C1, C2, . . . ,
Cn-1, and Cn, where C1 is a subclass of C2, C2 is a subclass of C3, . . . , and Cn-1 is a subclass of Cn,
as shown in Figure 11.2. That is, Cn is the most general class, and C1 is the most specific class. In
Java, Cn is the Object class. If o invokes a method p, the JVM searches for the implementation
of the method p in C1, C2, . . . , Cn-1, and Cn, in this order, until it is found. Once an implementation is found, the search stops and the first-found implementation is invoked.
Cn
Cn-1
java.lang.Object
C2
.....
C1
If o is an instance of C1, o is also an
instance of C2, C3, …, Cn-1, and Cn
FIGURE 11.2 The method to be invoked is dynamically bound at runtime.
VideoNote
Polymorphism and dynamic
binding demo
polymorphic call
dynamic binding
override toString()
Listing 11.6 gives an example to demonstrate dynamic binding.
LISTING 11.6 DynamicBindingDemo.java
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
public class DynamicBindingDemo {
public static void main(String[] args) {
m(new GraduateStudent());
m(new Student());
m(new Person());
m(new Object());
}
public static void m(Object x) {
System.out.println(x.toString());
}
}
class GraduateStudent extends Student {
}
class Student extends Person {
@Override
public String toString() {
11.8 Dynamic Binding 425
20
21
22
23
24
25
26
27
28
29
return "Student" ;
}
}
class Person extends Object {
@Override
public String toString() {
return "Person" ;
}
}
override toString()
Student
Student
Person
java.lang.Object@130c19b
Method m (line 9) takes a parameter of the Object type. You can invoke m with any object
(e.g., new GraduateStudent(), new Student(), new Person(), and new Object())
in lines 3–6).
When the method m(Object x) is executed, the argument x’s toString method is
invoked. x may be an instance of GraduateStudent, Student, Person, or Object. The
classes GraduateStudent, Student, Person, and Object have their own implementations
of the toString method. Which implementation is used will be determined by x’s actual type
at runtime. Invoking m(new GraduateStudent()) (line 3) causes the toString method
defined in the Student class to be invoked.
Invoking m(new Student()) (line 4) causes the toString method defined in the Student class to be invoked; invoking m(new Person()) (line 5) causes the toString method
defined in the Person class to be invoked; and invoking m(new Object()) (line 6) causes
the toString method defined in the Object class to be invoked.
Matching a method signature and binding a method implementation are two separate
issues. The declared type of the reference variable decides which method to match at compile time. The compiler finds a matching method according to the parameter type, number of
parameters, and order of the parameters at compile time. A method may be implemented in
several classes along the inheritance chain. The JVM dynamically binds the implementation
of the method at runtime, decided by the actual type of the variable.
11.17 What is polymorphism? What is dynamic binding?
11.18 Describe the difference between method matching and method binding.
11.19 Can you assign new int[50], new Integer[50], new String[50], or
Object[50], into a variable of Object[] type?
11.20 What is wrong in the following code?
1
2
3
4
5
6
7
8
9
10
11
12
public class Test {
public static void main(String[] args) {
Integer[] list1 = {12, 24, 55, 1};
Double[] list2 = {12.4, 24.0, 55.2, 1.0};
int[] list3 = {1, 2, 3};
printArray(list1);
printArray(list2);
printArray(list3);
}
public static void printArray(Object[] list) {
for (Object o: list)
matching vs. binding
✓
Check
Point
new
426 Chapter 11
Inheritance and Polymorphism
13
14
15
16
System.out.print(o + " ");
System.out.println();
}
}
11.21 Show the output of the following code:
public class Test {
public static void main(String[] args) {
new Person().printPerson();
new Student().printPerson();
}
}
public class Test {
public static void main(String[] args) {
new Person().printPerson();
new Student().printPerson();
}
}
class Student extends Person {
@Override
public String getInfo() {
return "Student";
}
}
class Student extends Person {
private String getInfo() {
return "Student";
}
}
class Person {
public String getInfo() {
return "Person";
}
public void printPerson() {
System.out.println(getInfo());
}
}
class Person {
private String getInfo() {
return "Person";
}
public void printPerson() {
System.out.println(getInfo());
}
}
(a)
(b)
11.22 Show the output of following program:
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
public class Test {
public static void main(String[] args) {
A a = new A(3);
}
}
class A extends B {
public A(int t) {
System.out.println("A's constructor is invoked");
}
}
class B {
public B() {
System.out.println("B's constructor is invoked");
}
}
Is the no-arg constructor of Object invoked when new A(3) is invoked?
11.23 Show the output of following program:
public class Test {
public static void main(String[] args) {
new A();
new B();
}
}
11.9 Casting Objects and the instanceof Operator 427
class A {
int i = 7;
public A() {
setI(20);
System.out.println("i from A is " + i);
}
public void setI(int i) {
this.i = 2 * i;
}
}
class B extends A {
public B() {
System.out.println("i from B is " + i);
}
public void setI(int i) {
this.i = 3 * i;
}
}
11.9 Casting Objects and the instanceof Operator
One object reference can be typecast into another object reference. This is called casting object.
In the preceding section, the statement
Key
Point
casting object
m(new Student());
assigns the object new Student() to a parameter of the Object type. This statement is
equivalent to
Object o = new Student(); // Implicit casting
m(o);
The statement Object o = new Student(), known as implicit casting, is legal because an
instance of Student is an instance of Object.
Suppose you want to assign the object reference o to a variable of the Student type using
the following statement:
implicit casting
Student b = o;
In this case a compile error would occur. Why does the statement Object o = new Student() work but Student b = o doesn’t? The reason is that a Student object is always an
instance of Object, but an Object is not necessarily an instance of Student. Even though
you can see that o is really a Student object, the compiler is not clever enough to know it. To
tell the compiler that o is a Student object, use explicit casting. The syntax is similar to the
explicit casting
one used for casting among primitive data types. Enclose the target object type in parentheses
and place it before the object to be cast, as follows:
Student b = (Student)o; // Explicit casting
It is always possible to cast an instance of a subclass to a variable of a superclass (known as
upcasting), because an instance of a subclass is always an instance of its superclass. When
casting an instance of a superclass to a variable of its subclass (known as downcasting), explicit
upcasting
downcasting
428 Chapter 11
ClassCastException
instanceof
Inheritance and Polymorphism
casting must be used to confirm your intention to the compiler with the (SubclassName)
cast notation. For the casting to be successful, you must make sure that the object to be cast is
an instance of the subclass. If the superclass object is not an instance of the subclass, a runtime
ClassCastException occurs. For example, if an object is not an instance of Student, it
cannot be cast into a variable of Student. It is a good practice, therefore, to ensure that the
object is an instance of another object before attempting a casting. This can be accomplished
by using the instanceof operator. Consider the following code:
Object myObject = new Circle();
... // Some lines of code
/** Perform casting if myObject is an instance of Circle */
if (myObject instanceof Circle) {
System.out.println("The circle diameter is " +
((Circle)myObject).getDiameter());
...
}
You may be wondering why casting is necessary. The variable myObject is declared
Object. The declared type decides which method to match at compile time. Using
myObject.getDiameter() would cause a compile error, because the Object
class does not have the getDiameter method. The compiler cannot find a match for
myObject.getDiameter(). Therefore, it is necessary to cast myObject into the Circle
type to tell the compiler that myObject is also an instance of Circle.
Why not define myObject as a Circle type in the first place? To enable generic programming, it is a good practice to define a variable with a supertype, which can accept an object
of any subtype.
Note
instanceof is a Java keyword. Every letter in a Java keyword is in lowercase.
lowercase keywords
Tip
To help understand casting, you may also consider the analogy of fruit, apple, and
orange, with the Fruit class as the superclass for Apple and Orange. An apple is
a fruit, so you can always safely assign an instance of Apple to a variable for Fruit.
However, a fruit is not necessarily an apple, so you have to use explicit casting to assign
an instance of Fruit to a variable of Apple.
casting analogy
Listing 11.7 demonstrates polymorphism and casting. The program creates two objects
(lines 5–6), a circle and a rectangle, and invokes the displayObject method to display them
(lines 9–10). The displayObject method displays the area and diameter if the object is a
circle (line 15), and the area if the object is a rectangle (lines 21–22).
LISTING 11.7 CastingDemo.java
1
2
3
4
5
6
7
8
9
10
11
12
public class CastingDemo {
/** Main method */
public static void main(String[] args) {
// Create and initialize two objects
Object object1 = new CircleFromSimpleGeometricObject(1);
Object object2 = new RectangleFromSimpleGeometricObject(1, 1);
// Display circle and rectangle
displayObject(object1);
displayObject(object2);
}
11.9 Casting Objects and the instanceof Operator 429
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
/** A method for displaying an object */
public static void displayObject(Object object) {
if (object instanceof CircleFromSimpleGeometricObject) {
System.out.println("The circle area is " +
((CircleFromSimpleGeometricObject)object).getArea());
System.out.println("The circle diameter is " +
((CircleFromSimpleGeometricObject)object).getDiameter());
}
else if (object instanceof
RectangleFromSimpleGeometricObject) {
System.out.println("The rectangle area is " +
((RectangleFromSimpleGeometricObject)object).getArea());
}
}
polymorphic call
polymorphic call
}
The circle area is 3.141592653589793
The circle diameter is 2.0
The rectangle area is 1.0
The displayObject(Object object) method is an example of generic programming. It
can be invoked by passing any instance of Object.
The program uses implicit casting to assign a Circle object to object1 and a Rectangle
object to object2 (lines 5–6), then invokes the displayObject method to display the information on these objects (lines 9–10).
In the displayObject method (lines 14–26), explicit casting is used to cast the object to
Circle if the object is an instance of Circle, and the methods getArea and getDiameter
are used to display the area and diameter of the circle.
Casting can be done only when the source object is an instance of the target class. The
program uses the instanceof operator to ensure that the source object is an instance of the
target class before performing a casting (line 15).
Explicit casting to Circle (lines 17, 19) and to Rectangle (line 24) is necessary because
the getArea and getDiameter methods are not available in the Object class.
Caution
The object member access operator (.) precedes the casting operator. Use parentheses
to ensure that casting is done before the . operator, as in
((Circle)object).getArea();
Casting a primitive type value is different from casting an object reference. Casting a primitive type value returns a new value. For example:
int age = 45;
byte newAge = (byte)age; // A new value is assigned to newAge
However, casting an object reference does not create a new object. For example:
Object o = new Circle();
Circle c = (Circle)o; // No new object is created
Now reference variables o and c point to the same object.
precedes casting
430 Chapter 11
✓
Inheritance and Polymorphism
Check
Point
11.24 Indicate true or false for the following statements:
■
You can always successfully cast an instance of a subclass to a superclass.
■
You can always successfully cast an instance of a superclass to a subclass.
11.25 For the GeometricObject and Circle classes in Listings 11.1 and 11.2, answer the
following questions:
a. Assume are circle and object created as follows:
Circle circle = new Circle(1);
GeometricObject object = new GeometricObject();
Are the following Boolean expressions true or false?
(circle
(object
(circle
(object
instanceof
instanceof
instanceof
instanceof
GeometricObject)
GeometricObject)
Circle)
Circle)
b. Can the following statements be compiled?
Circle circle = new Circle(5);
GeometricObject object = circle;
c. Can the following statements be compiled?
GeometricObject object = new GeometricObject();
Circle circle = (Circle)object;
11.26 Suppose that
Fruit, Apple, Orange, GoldenDelicious, and McIntosh are
defined in the following inheritance hierarchy:
Fruit
Apple
GoldenDelicious
Orange
McIntosh
Assume that the following code is given:
Fruit fruit = new GoldenDelicious();
Orange orange = new Orange();
Answer the following questions:
a. Is fruit instanceof Fruit?
b. Is fruit instanceof Orange?
c. Is fruit instanceof Apple?
d. Is fruit instanceof GoldenDelicious?
e. Is fruit instanceof McIntosh?
f. Is orange instanceof Orange?
11.10 The Object’s equals Method 431
g. Is orange instanceof Fruit?
h. Is orange instanceof Apple?
i. Suppose the method makeAppleCider is defined in the Apple class. Can fruit
invoke this method? Can orange invoke this method?
j. Suppose the method makeOrangeJuice is defined in the Orange class. Can
orange invoke this method? Can fruit invoke this method?
k. Is the statement Orange p = new Apple() legal?
l. Is the statement McIntosh p = new Apple() legal?
m. Is the statement Apple p = new McIntosh() legal?
11.27 What is wrong in the following code?
1
2
3
4
5
6
7
8
9
10
11
12
public class Test {
public static void main(String[] args) {
Object fruit = new Fruit();
Object apple = (Apple)fruit;
}
}
class Apple extends Fruit {
}
class Fruit {
}
11.10 The Object’s equals Method
Like the toString() method, the equals(Object) method is another useful
method defined in the Object class.
Another method defined in the Object class that is often used is the equals method. Its
signature is
public boolean equals(Object o)
This method tests whether two objects are equal. The syntax for invoking it is:
object1.equals(object2);
The default implementation of the equals method in the Object class is:
public boolean equals(Object obj) {
return (this == obj);
}
This implementation checks whether two reference variables point to the same object using
the == operator. You should override this method in your custom class to test whether two
distinct objects have the same content.
The equals method is overridden in many classes in the Java API, such as java.lang
.String and java.util.Date, to compare whether the contents of two objects are equal.
You have already used the equals method to compare two strings in Section 4.4.7, The
String Class. The equals method in the String class is inherited from the Object class
and is overridden in the String class to test whether two strings are identical in content.
Key
Point
432 Chapter 11
Inheritance and Polymorphism
You can override the equals method in the Circle class to compare whether two circles
are equal based on their radius as follows:
public boolean equals(Object o) {
if (o instanceof Circle)
return radius == ((Circle)o).radius;
else
return this == o;
}
Note
The == comparison operator is used for comparing two primitive data type values or
for determining whether two objects have the same references. The equals method is
intended to test whether two objects have the same contents, provided that the method
is overridden in the defining class of the objects. The == operator is stronger than the
equals method, in that the == operator checks whether the two reference variables
refer to the same object.
== vs. equals
Caution
Using the signature equals(SomeClassName obj) (e.g., equals(Circle c))
to override the equals method in a subclass is a common mistake. You should use
equals(Object obj). See CheckPoint Question 11.29.
equals(Object)
✓
Check
Point
11.28 Does every object have a toString method and an equals method? Where do they
come from? How are they used? Is it appropriate to override these methods?
11.29 When overriding the equals method, a common mistake is mistyping its signature
in the subclass. For example, the equals method is incorrectly written as
equals(Circle circle), as shown in (a) in following the code; instead, it should
be equals(Object circle), as shown in (b). Show the output of running class
Test with the Circle class in (a) and in (b), respectively.
public class Test {
public static void main(String[] args) {
Object circle1 = new Circle();
Object circle2 = new Circle();
System.out.println(circle1.equals(circle2));
}
}
class Circle {
double radius;
class Circle {
double radius;
public boolean equals(Circle circle) {
return this.radius == circle.radius;
}
public boolean equals(Object circle) {
return this.radius ==
((Circle)circle).radius;
}
}
}
(a)
(b)
If Object is replaced by Circle in the Test class, what would be the output to
run Test using the Circle class in (a) and (b), respectively?
11.11 The ArrayList Class
VideoNote
The ArrayList class
Key
Point
An ArrayList object can be used to store a list of objects.
Now we are ready to introduce a very useful class for storing objects. You can create an array
to store objects. But, once the array is created, its size is fixed. Java provides the ArrayList
11.11 The ArrayList Class 433
class, which can be used to store an unlimited number of objects. Figure 11.3 shows some
methods in ArrayList.
java.util.ArrayList<E>
+ArrayList()
Creates an empty list.
+add(o: E): void
Appends a new element o at the end of this list.
+add(index: int, o: E): void
Adds a new element o at the specified index in this list.
+clear(): void
Removes all the elements from this list.
+contains(o: Object): boolean
Returns true if this list contains the element o.
+get(index: int): E
Returns the element from this list at the specified index.
+indexOf(o: Object): int
Returns the index of the first matching element in this list.
+isEmpty(): boolean
Returns true if this list contains no elements.
+lastIndexOf(o: Object): int
Returns the index of the last matching element in this list.
+remove(o: Object): boolean
Removes the first element o from this list. Returns true
if an element is removed.
+size(): int
+remove(index: int): boolean
Returns the number of elements in this list.
Removes the element at the specified index. Returns true
if an element is removed.
+set(index: int, o: E): E
Sets the element at the specified index.
FIGURE 11.3 An ArrayList stores an unlimited number of objects.
ArrayList is known as a generic class with a generic type E. You can specify a concrete
type to replace E when creating an ArrayList. For example, the following statement creates
an ArrayList and assigns its reference to variable cities. This ArrayList object can be
used to store strings.
ArrayList<String> cities = new ArrayList<String>();
The following statement creates an ArrayList and assigns its reference to variable
dates. This ArrayList object can be used to store dates.
ArrayList<java.util.Date> dates = new ArrayList<java.util.Date> ();
Note
Since JDK 7, the statement
ArrayList<AConcreteType> list = new ArrayList<AConcreteType>();
can be simplified by
ArrayList<AConcreteType> list = new ArrayList<>();
The concrete type is no longer required in the constructor thanks to a feature called
type inference. The compiler is able to infer the type from the variable declaration. More
discussions on generics including how to define custom generic classes and methods
will be introduced in Chapter 19, Generics.
type inference
Listing 11.8 gives an example of using ArrayList to store objects.
LISTING 11.8
1
2
TestArrayList.java
import java.util.ArrayList;
import ArrayList
434 Chapter 11
create ArrayList
add element
list size
contains element?
element index
is empty?
remove element
remove element
toString()
get element
create ArrayList
Inheritance and Polymorphism
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
public class TestArrayList {
public static void main(String[] args) {
// Create a list to store cities
ArrayList<String> cityList = new ArrayList<>();
// Add some cities in the list
cityList.add("London");
// cityList now contains [London]
cityList.add("Denver");
// cityList now contains [London, Denver]
cityList.add("Paris");
// cityList now contains [London, Denver, Paris]
cityList.add("Miami");
// cityList now contains [London, Denver, Paris, Miami]
cityList.add("Seoul");
// Contains [London, Denver, Paris, Miami, Seoul]
cityList.add("Tokyo");
// Contains [London, Denver, Paris, Miami, Seoul, Tokyo]
System.out.println("List size? " + cityList.size());
System.out.println("Is Miami in the list? " +
cityList.contains("Miami"));
System.out.println("The location of Denver in the list? "
+ cityList.indexOf("Denver"));
System.out.println("Is the list empty? " +
cityList.isEmpty()); // Print false
// Insert a new city at index 2
cityList.add(2, "Xian");
// Contains [London, Denver, Xian, Paris, Miami, Seoul, Tokyo]
// Remove a city from the list
cityList.remove("Miami");
// Contains [London, Denver, Xian, Paris, Seoul, Tokyo]
// Remove a city at index 1
cityList.remove(1);
// Contains [London, Xian, Paris, Seoul, Tokyo]
// Display the contents in the list
System.out.println(cityList.toString());
// Display the contents in the list in reverse order
for (int i = cityList.size() - 1; i >= 0; i––)
System.out.print(cityList.get(i) + " ");
System.out.println();
// Create a list to store two circles
ArrayList<CircleFromSimpleGeometricObject> list
= new ArrayList<>();
// Add two circles
list.add(new CircleFromSimpleGeometricObject(2));
list.add(new CircleFromSimpleGeometricObject(3));
// Display the area of the first circle in the list
System.out.println("The area of the circle? " +
list.get(0).getArea());
}
}
11.11 The ArrayList Class 435
List size? 6
Is Miami in the list? True
The location of Denver in the list? 1
Is the list empty? false
[London, Xian, Paris, Seoul, Tokyo]
Tokyo Seoul Paris Xian London
The area of the circle? 12.566370614359172
Since the ArrayList is in the java.util package, it is imported in line 1. The program
creates an ArrayList of strings using its no-arg constructor and assigns the reference to
cityList (line 6). The add method (lines 9–19) adds strings to the end of list. So, after
cityList.add("London") (line 9), the list contains
add(Object)
[London]
After cityList.add("Denver") (line 11), the list contains
[London, Denver]
After adding Paris, Miami, Seoul, and Tokyo (lines 13–19), the list contains
[London, Denver, Paris, Miami, Seoul, Tokyo]
Invoking size() (line 22) returns the size of the list, which is currently 6. Invoking
contains("Miami") (line 24) checks whether the object is in the list. In this case, it returns
true, since Miami is in the list. Invoking indexOf("Denver") (line 26) returns the index of
Denver in the list, which is 1. If Denver were not in the list, it would return -1. The isEmpty()
method (line 28) checks whether the list is empty. It returns false, since the list is not empty.
The statement cityList.add(2, "Xian") (line 31) inserts an object into the list at the
specified index. After this statement, the list becomes
size()
add(index, Object)
[London, Denver, Xian, Paris, Miami, Seoul, Tokyo]
The statement cityList.remove("Miami") (line 35) removes the object from the list.
After this statement, the list becomes
remove(Object)
[London, Denver, Xian, Paris, Seoul, Tokyo]
The statement cityList.remove(1) (line 39) removes the object at the specified index
from the list. After this statement, the list becomes
remove(index)
[London, Xian, Paris, Seoul, Tokyo]
The statement in line 43 is same as
System.out.println(cityList);
The toString() method returns a string representation of the list in the form of
[e0.toString(), e1.toString(), ..., ek.toString()], where e0, e1, . . . , and
ek are the elements in the list.
The get(index) method (line 47) returns the object at the specified index.
ArrayList objects can be used like arrays, but there are many differences. Table 11.1 lists
their similarities and differences.
Once an array is created, its size is fixed. You can access an array element using the
square-bracket notation (e.g., a[index]). When an ArrayList is created, its size is 0.
toString()
get(index)
array vs. ArrayList
436 Chapter 11
Inheritance and Polymorphism
TABLE 11.1 Differences and Similarities between Arrays and ArrayList
Operation
Array
ArrayList
Creating an array/ArrayList
String[] a = new String[10]
ArrayList<String> list = new ArrayList<>();
Accessing an element
a[index]
list.get(index);
Updating an element
a[index] = "London";
list.set(index, "London");
Returning size
a.length
list.size();
Adding a new element
list.add("London");
Inserting a new element
list.add(index, "London");
Removing an element
list.remove(index);
Removing an element
list.remove(Object);
Removing all elements
list.clear();
You cannot use the get(index) and set(index, element) methods if the element is not
in the list. It is easy to add, insert, and remove elements in a list, but it is rather complex to
add, insert, and remove elements in an array. You have to write code to manipulate the array
in order to perform these operations. Note that you can sort an array using the java.util.
Arrays.sort(array) method. To sort an array list, use the java.util.Collections.
sort(arraylist) method.
Suppose you want to create an ArrayList for storing integers. Can you use the following
code to create a list?
ArrayList<int> list = new ArrayList<>();
No. This will not work because the elements stored in an ArrayList must be of an object
type. You cannot use a primitive data type such as int to replace a generic type. However,
you can create an ArrayList for storing Integer objects as follows:
ArrayList<Integer> list = new ArrayList<>();
Listing 11.9 gives a program that prompts the user to enter a sequence of numbers and
displays the distinct numbers in the sequence. Assume that the input ends with 0 and 0 is not
counted as a number in the sequence.
LISTING 11.9 DistinctNumbers.java
create an array list
contained in list?
add to list
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
import java.util.ArrayList;
import java.util.Scanner;
public class DistinctNumbers {
public static void main(String[] args) {
ArrayList<Integer> list = new ArrayList<>();
Scanner input = new Scanner(System.in);
System.out.print("Enter integers (input ends with 0): ");
int value;
do {
value = input.nextInt(); // Read a value from the input
if (!list.contains(value) && value != 0)
list.add(value); // Add the value if it is not in the list
} while (value != 0);
11.11 The ArrayList Class 437
18
19
20
21
22
23
// Display the distinct numbers
for (int i = 0; i < list.size(); i++)
System.out.print(list.get(i) + " ");
}
}
Enter numbers (input ends with 0): 1 2 3 2 1 6 3 4 5 4 5 1 2 3 0
The distinct numbers are: 1 2 3 6 4 5
The program creates an ArrayList for Integer objects (line 6) and repeatedly reads a value in
the loop (lines 12–17). For each value, if it is not in the list (line 15), add it to the list (line 16). You
can rewrite this program using an array to store the elements rather than using an ArrayList.
However, it is simpler to implement this program using an ArrayList for two reasons.
■
First, the size of an ArrayList is flexible so you don’t have to specify its size in
advance. When creating an array, its size must be specified.
■
Second, ArrayList contains many useful methods. For example, you can test
whether an element is in the list using the contains method. If you use an array,
you have to write additional code to implement this method.
You can traverse the elements in an array using a foreach loop. The elements in an array list
can also be traversed using a foreach loop using the following syntax:
for (elementType element: arrayList) {
// Process the element
}
For example, you can replace the code in lines 20-21 using the following code:
for (int number: list)
System.out.print(number + “ “);
11.30 How do you do the following?
a. Create an ArrayList for storing double values?
b. Append an object to a list?
c. Insert an object at the beginning of a list?
d. Find the number of objects in a list?
e. Remove a given object from a list?
f. Remove the last object from the list?
g. Check whether a given object is in a list?
h. Retrieve an object at a specified index from a list?
11.31 Identify the errors in the following code.
ArrayList<String> list = new ArrayList<>();
list.add("Denver");
list.add("Austin");
list.add(new java.util.Date());
String city = list.get(0);
list.set(3, "Dallas");
System.out.println(list.get(3));
✓
Check
Point
438 Chapter 11
Inheritance and Polymorphism
11.32 Suppose the ArrayList
list contains {"Dallas", "Dallas", "Houston",
"Dallas"}. What is the list after invoking list.remove("Dallas") one time?
Does the following code correctly remove all elements with value "Dallas" from
the list? If not, correct the code.
for (int i = 0; i < list.size(); i++)
list.remove("Dallas");
11.33 Explain why the following code displays [1,
3] rather than [2, 3].
ArrayList<Integer> list = new ArrayList<>();
list.add(1);
list.add(2);
list.add(3);
list.remove(1);
System.out.println(list);
11.34 Explain why the following code is wrong.
ArrayList<Double> list = new ArrayList<>();
list.add(1);
11.12 Useful Methods for Lists
Key
Point
array to array list
Java provides the methods for creating a list from an array, for sorting a list, and
finding maximum and minimum element in a list, and for shuffling a list.
Often you need to create an array list from an array of objects or vice versa. You can write the
code using a loop to accomplish this, but an easy way is to use the methods in the Java API.
Here is an example to create an array list from an array:
String[] array = {"red", "green", "blue"};
ArrayList<String> list = new ArrayList<>(Arrays.asList(array));
array list to array
The static method asList in the Arrays class returns a list that is passed to the ArrayList
constructor for creating an ArrayList. Conversely, you can use the following code to create
an array of objects from an array list.
String[] array1 = new String[list.size()];
list.toArray(array1);
sort a list
Invoking list.toArray(array1) copies the contents from list to array1.
If the elements in a list are comparable such as integers, double, or strings, you can use the
static sort method in the java.util.Collections class to sort the elements. Here are
examples:
Integer[] array = {3, 5, 95, 4, 15, 34, 3, 6, 5};
ArrayList<Integer> list = new ArrayList<>(Arrays.asList(array));
java.util.Collections.sort(list);
System.out.println(list);
max and min methods
You can use the static max and min in the java.util.Collections class to return the
maximum and minimal element in a list. Here are examples:
Integer[] array = {3, 5, 95, 4, 15, 34, 3, 6, 5};
ArrayList<Integer> list = new ArrayList<>(Arrays.asList(array));
System.out.println(java.util.Collections.max(list));
System.out.println(java.util.Collections.min(list));
11.13 Case Study: A Custom Stack Class 439
You can use the static shuffle method in the java.util.Collections class to perform
a random shuffle for the elements in a list. Here are examples:
shuffle method
Integer[] array = {3, 5, 95, 4, 15, 34, 3, 6, 5};
ArrayList<Integer> list = new ArrayList<>(Arrays.asList(array));
java.util.Collections.shuffle(list);
System.out.println(list);
11.35 Correct errors in the following statements:
int[] array = {3, 5, 95, 4, 15, 34, 3, 6, 5};
ArrayList<Integer> list = new ArrayList<>(Arrays.asList(array));
✓
Check
Point
11.36 Correct errors in the following statements:
int[] array = {3, 5, 95, 4, 15, 34, 3, 6, 5};
System.out.println(java.util.Collections.max(array));
11.13 Case Study: A Custom Stack Class
This section designs a stack class for holding objects.
Section 10.6 presented a stack class for storing int values. This section introduces a stack class
to store objects. You can use an ArrayList to implement Stack, as shown in Listing 11.10.
The UML diagram for the class is shown in Figure 11.4.
Key
Point
VideoNote
The MyStack class
MyStack
-list: ArrayList<Object>
A list to store elements.
+isEmpty(): boolean
Returns true if this stack is empty.
+getSize(): int
Returns the number of elements in this stack.
+peek(): Object
Returns the top element in this stack without removing it.
+pop(): Object
Returns and removes the top element in this stack.
+push(o: Object): void
Adds a new element to the top of this stack.
FIGURE 11.4 The MyStack class encapsulates the stack storage and provides the operations
for manipulating the stack.
LISTING 11.10
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
MyStack.java
import java.util.ArrayList;
public class MyStack {
private ArrayList<Object> list = new ArrayList<>();
array list
public boolean isEmpty() {
return list.isEmpty();
}
stack empty?
public int getSize() {
return list.size();
}
get stack size
public Object peek() {
return list.get(getSize() - 1);
}
peek stack
440 Chapter 11
Inheritance and Polymorphism
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
remove
push
public Object pop() {
Object o = list.get(getSize() - 1);
list.remove(getSize() - 1);
return o;
}
public void push(Object o) {
list.add(o);
}
@Override
public String toString() {
return "stack: " + list.toString();
}
}
An array list is created to store the elements in the stack (line 4). The isEmpty() method (lines
6–8) returns list.isEmpty(). The getSize() method (lines 10–12) returns list.size().
The peek() method (lines 14–16) retrieves the element at the top of the stack without removing it. The end of the list is the top of the stack. The pop() method (lines 18–22) removes the
top element from the stack and returns it. The push(Object element) method (lines 24–26)
adds the specified element to the stack. The toString() method (lines 28–31) defined in the
Object class is overridden to display the contents of the stack by invoking list.toString().
The toString() method implemented in ArrayList returns a string representation of all the
elements in an array list.
Design Guide
In Listing 11.10, MyStack contains ArrayList. The relationship between MyStack
and ArrayList is composition. While inheritance models an is-a relationship, composition models a has-a relationship. You could also implement MyStack as a subclass of
ArrayList (see Programming Exercise 11.10). Using composition is better, however,
because it enables you to define a completely new stack class without inheriting the
unnecessary and inappropriate methods from ArrayList.
composition
is-a
has-a
11.14 The protected Data and Methods
Key
Point
why protected?
A protected member of a class can be accessed from a subclass.
So far you have used the private and public keywords to specify whether data fields and
methods can be accessed from outside of the class. Private members can be accessed only
from inside of the class, and public members can be accessed from any other classes.
Often it is desirable to allow subclasses to access data fields or methods defined in the
superclass, but not to allow nonsubclasses to access these data fields and methods. To accomplish this, you can use the protected keyword. This way you can access protected data
fields or methods in a superclass from its subclasses.
The modifiers private, protected, and public are known as visibility or accessibility
modifiers because they specify how classes and class members are accessed. The visibility of
these modifiers increases in this order:
Visibility increases
private, default (no modifier), protected, public
Table 11.2 summarizes the accessibility of the members in a class. Figure 11.5 illustrates how
a public, protected, default, and private datum or method in class C1 can be accessed from a
class C2 in the same package, from a subclass C3 in the same package, from a subclass C4 in
a different package, and from a class C5 in a different package.
11.14 The protected Data and Methods 441
Use the private modifier to hide the members of the class completely so that they cannot
be accessed directly from outside the class. Use no modifiers (the default) in order to allow
the members of the class to be accessed directly from any class within the same package but
not from other packages. Use the protected modifier to enable the members of the class to
be accessed by the subclasses in any package or classes in the same package. Use the public
modifier to enable the members of the class to be accessed by any class.
TABLE 11.2 Data and Methods Visibility
Modifier
on members
in a class
Accessed
from the
same class
Accessed
from the
same package
Accessed from
a subclass in a
different package
Accessed
from a different
package
public
✓
✓
✓
✓
protected
✓
✓
✓
–
default (no modifier)
✓
✓
–
–
private
✓
–
–
–
package p1;
public class C1 {
public int x;
protected int y;
int z;
private int u;
public class C2 {
C1 o = new C1();
can access o.x;
can access o.y;
can access o.z;
cannot access o.u;
protected void m() {
}
}
can invoke o.m();
}
package p2;
public class C3
extends C1 {
can access x;
can access y;
can access z;
cannot access u;
public class C4
extends C1 {
can access x;
can access y;
cannot access z;
cannot access u;
can invoke m();
}
public class C5 {
C1 o = new C1();
can access o.x;
cannot access o.y;
cannot access o.z;
cannot access o.u;
can invoke m();
}
cannot invoke o.m();
}
FIGURE 11.5 Visibility modifiers are used to control how data and methods are accessed.
Your class can be used in two ways: (1) for creating instances of the class and (2) for defining subclasses by extending the class. Make the members private if they are not intended
for use from outside the class. Make the members public if they are intended for the users of
the class. Make the fields or methods protected if they are intended for the extenders of the
class but not for the users of the class.
The private and protected modifiers can be used only for members of the class.
The public modifier and the default modifier (i.e., no modifier) can be used on members of
the class as well as on the class. A class with no modifier (i.e., not a public class) is not accessible by classes from other packages.
442 Chapter 11
Inheritance and Polymorphism
Note
A subclass may override a protected method defined in its superclass and change its
visibility to public. However, a subclass cannot weaken the accessibility of a method
defined in the superclass. For example, if a method is defined as public in the superclass,
it must be defined as public in the subclass.
change visibility
✓
Check
Point
11.37 What modifier should you use on a class so that a class in the same package can
access it, but a class in a different package cannot access it?
11.38 What modifier should you use so that a class in a different package cannot access the
class, but its subclasses in any package can access it?
11.39 In the following code, the classes A and B are in the same package. If the question
marks in (a) are replaced by blanks, can class B be compiled? If the question marks
are replaced by private, can class B be compiled? If the question marks are replaced
by protected, can class B be compiled?
package p1;
package p1;
public class A {
?
int i;
public class B extends A {
public void m1(String[] args) {
System.out.println(i);
m();
}
}
?
...
void m() {
}
}
(a)
(b)
11.40 In the following code, the classes A and B are in different packages. If the question
marks in (a) are replaced by blanks, can class B be compiled? If the question marks
are replaced by private, can class B be compiled? If the question marks are replaced
by protected, can class B be compiled?
package p1;
package p2;
public class A {
?
int i;
public class B extends A {
public void m1(String[] args) {
System.out.println(i);
m();
}
}
?
...
void m() {
}
}
(a)
(b)
11.15 Preventing Extending and Overriding
Key
Point
Neither a final class nor a final method can be extended. A final data field is a
constant.
You may occasionally want to prevent classes from being extended. In such cases, use the
final modifier to indicate that a class is final and cannot be a parent class. The Math class
is a final class. The String, StringBuilder, and StringBuffer classes are also final
classes. For example, the following class A is final and cannot be extended:
public final class A {
// Data fields, constructors, and methods omitted
}
Chapter Summary 443
You also can define a method to be final; a final method cannot be overridden by its
subclasses.
For example, the following method m is final and cannot be overridden:
public class Test {
// Data fields, constructors, and methods omitted
public final void m() {
// Do something
}
}
Note
The modifiers public, protected, private, static, abstract, and final are
used on classes and class members (data and methods), except that the final modifier
can also be used on local variables in a method. A final local variable is a constant
inside a method.
11.41 How do you prevent a class from being extended? How do you prevent a method from
being overridden?
11.42 Indicate true or false for the following statements:
a. A protected datum or method can be accessed by any class in the same package.
b. A protected datum or method can be accessed by any class in different packages.
c. A protected datum or method can be accessed by its subclasses in any package.
d. A final class can have instances.
e. A final class can be extended.
f. A final method can be overridden.
KEY TERMS
actual type 424
casting objects 427
constructor chaining 417
declared type 424
dynamic binding 424
inheritance 410
instanceof 428
is-a relationship 440
method overriding 419
multiple inheritance 416
override 000
polymorphism
423
440
single inheritance 416
subclass 410
subtype 423
superclass 410
supertype 423
type inference 433
protected
CHAPTER SUMMARY
1. You can define a new class from an existing class. This is known as class inheritance.
The new class is called a subclass, child class, or extended class. The existing class is
called a superclass, parent class, or base class.
2. A constructor is used to construct an instance of a class. Unlike properties and methods,
the constructors of a superclass are not inherited in the subclass. They can be invoked
only from the constructors of the subclasses, using the keyword super.
✓
Check
Point
444 Chapter 11
Inheritance and Polymorphism
3. A constructor may invoke an overloaded constructor or its superclass’s constructor. The
call must be the first statement in the constructor. If none of them is invoked explicitly,
the compiler puts super() as the first statement in the constructor, which invokes the
superclass’s no-arg constructor.
4. To override a method, the method must be defined in the subclass using the same signature and the same return type as in its superclass.
5. An instance method can be overridden only if it is accessible. Thus, a private method
cannot be overridden because it is not accessible outside its own class. If a method
defined in a subclass is private in its superclass, the two methods are completely
unrelated.
6. Like an instance method, a static method can be inherited. However, a static method
cannot be overridden. If a static method defined in the superclass is redefined in a subclass, the method defined in the superclass is hidden.
7. Every class in Java is descended from the java.lang.Object class. If no superclass
is specified when a class is defined, its superclass is Object.
8. If a method’s parameter type is a superclass (e.g., Object), you may pass an object
to this method of any of the parameter’s subclasses (e.g., Circle or String). This is
known as polymorphism.
9. It is always possible to cast an instance of a subclass to a variable of a superclass,
because an instance of a subclass is always an instance of its superclass. When casting an instance of a superclass to a variable of its subclass, explicit casting must
be used to confirm your intention to the compiler with the (SubclassName) cast
notation.
10. A class defines a type. A type defined by a subclass is called a subtype and a type
defined by its superclass is called a supertype.
11. When invoking an instance method from a reference variable, the actual type of the
variable decides which implementation of the method is used at runtime. This is known
as dynamic binding.
12. You can use obj
instanceof AClass to test whether an object is an instance of a
class.
13. You can use the ArrayList class to create an object to store a list of objects.
14. You can use the
protected modifier to prevent the data and methods from being
accessed by nonsubclasses from a different package.
15. You can use the final modifier to indicate that a class is final and cannot be extended
and to indicate that a method is final and cannot be overridden.
QUIZ
Answer the quiz for this chapter online at www.cs.armstrong.edu/liang/intro10e/quiz.html.
Programming Exercises 445
PROGRAMMING EXERCISES
Sections 11.2–11.4
11.1
(The Triangle class) Design a class named Triangle that extends
GeometricObject. The class contains:
■
■
■
■
■
■
■
Three double data fields named side1, side2, and side3 with default
values 1.0 to denote three sides of the triangle.
A no-arg constructor that creates a default triangle.
A constructor that creates a triangle with the specified side1, side2, and
side3.
The accessor methods for all three data fields.
A method named getArea() that returns the area of this triangle.
A method named getPerimeter() that returns the perimeter of this triangle.
A method named toString() that returns a string description for the triangle.
For the formula to compute the area of a triangle, see Programming Exercise 2.19.
The toString() method is implemented as follows:
return "Triangle: side1 = " + side1 + " side2 = " + side2 +
" side3 = " + side3;
Draw the UML diagrams for the classes Triangle and GeometricObject and
implement the classes. Write a test program that prompts the user to enter three
sides of the triangle, a color, and a Boolean value to indicate whether the triangle
is filled. The program should create a Triangle object with these sides and set
the color and filled properties using the input. The program should display
the area, perimeter, color, and true or false to indicate whether it is filled or not.
Sections 11.5–11.14
11.2
11.3
11.4
(The Person, Student, Employee, Faculty, and Staff classes) Design a
class named Person and its two subclasses named Student and Employee.
Make Faculty and Staff subclasses of Employee. A person has a name,
address, phone number, and email address. A student has a class status (freshman,
sophomore, junior, or senior). Define the status as a constant. An employee has
an office, salary, and date hired. Use the MyDate class defined in Programming
Exercise 10.14 to create an object for date hired. A faculty member has office
hours and a rank. A staff member has a title. Override the toString method in
each class to display the class name and the person’s name.
Draw the UML diagram for the classes and implement them. Write a test program that creates a Person, Student, Employee, Faculty, and Staff, and
invokes their toString() methods.
(Subclasses of Account) In Programming Exercise 9.7, the Account class was
defined to model a bank account. An account has the properties account number,
balance, annual interest rate, and date created, and methods to deposit and withdraw funds. Create two subclasses for checking and saving accounts. A checking
account has an overdraft limit, but a savings account cannot be overdrawn.
Draw the UML diagram for the classes and then implement them. Write
a test program that creates objects of Account, SavingsAccount, and
CheckingAccount and invokes their toString() methods.
(Maximum element in ArrayList) Write the following method that returns the
maximum value in an ArrayList of integers. The method returns null if the
list is null or the list size is 0.
public static Integer max(ArrayList<Integer> list)
446 Chapter 11
Inheritance and Polymorphism
Write a test program that prompts the user to enter a sequence of numbers ending
with 0, and invokes this method to return the largest number in the input.
11.5
(The Course class) Rewrite the Course class in Listing 10.6. Use an ArrayList
to replace an array to store students. Draw the new UML diagram for the class.
You should not change the original contract of the Course class (i.e., the definition of the constructors and methods should not be changed, but the private
members may be changed.)
11.6
(Use ArrayList) Write a program that creates an ArrayList and adds a Loan
object, a Date object, a string, and a Circle object to the list, and use a loop
to display all the elements in the list by invoking the object’s toString()
method.
(Shuffle ArrayList) Write the following method that shuffles the elements in
an ArrayList of integers.
11.7
public static void shuffle(ArrayList<Integer> list)
**11.8
VideoNote
New Account class
(New Account class) An Account class was specified in Programming
Exercise 9.7. Design a new Account class as follows:
■
■
■
Add a new data field name of the String type to store the name of the
customer.
Add a new constructor that constructs an account with the specified name, id,
and balance.
Add a new data field named transactions whose type is ArrayList
that stores the transaction for the accounts. Each transaction is an instance
of the Transaction class. The Transaction class is defined as shown in
Figure 11.6.
The getter and setter methods for these data
fields are provided in the class, but omitted in the UML
diagram for brevity.
Transaction
-date: java.util.Date
The date of this transaction.
-type: char
The type of the transaction, such as 'W' for withdrawal, 'D'
for deposit.
-amount: double
The amount of the transaction.
-balance: double
The new balance after this transaction.
-description: String
The description of this transaction.
+Transaction(type: char,
amount: double, balance:
double, description: String)
Construct a Transaction with the specified date, type,
balance, and description.
FIGURE 11.6 The Transaction class describes a transaction for a bank account.
■
■
Modify the withdraw and deposit methods to add a transaction to the
transactions array list.
All other properties and methods are the same as in Programming Exercise 9.7.
Write a test program that creates an Account with annual interest rate 1.5%,
balance 1000, id 1122, and name George. Deposit $30, $40, and $50 to the
account and withdraw $5, $4, and $2 from the account. Print an account summary that shows account holder name, interest rate, balance, and all transactions.
Programming Exercises 447
*11.9
(Largest rows and columns) Write a program that randomly fills in 0s and 1s
into an n-by-n matrix, prints the matrix, and finds the rows and columns with the
most 1s. (Hint: Use two ArrayLists to store the row and column indices with
the most 1s.) Here is a sample run of the program:
Enter the array size n: 4
The random array is
0011
0011
1101
1010
The largest row index: 2
The largest column index: 2, 3
11.10
11.11
(Implement MyStack using inheritance) In Listing 11.10, MyStack is implemented using composition. Define a new stack class that extends ArrayList.
Draw the UML diagram for the classes and then implement MyStack. Write
a test program that prompts the user to enter five strings and displays them in
reverse order.
(Sort ArrayList) Write the following method that sorts an ArrayList of
numbers:
public static void sort(ArrayList<Integer> list)
11.12
Write a test program that prompts the user to enter 5 numbers, stores them in an
array list, and displays them in increasing order.
(Sum ArrayList) Write the following method that returns the sum of all numbers in an ArrayList:
public static double sum(ArrayList<Double> list)
*11.13
Write a test program that prompts the user to enter 5 numbers, stores them in an
array list, and displays their sum.
(Remove duplicates) Write a method that removes the duplicate elements from
an array list of integers using the following header:
public static void removeDuplicate(ArrayList<Integer> list)
Write a test program that prompts the user to enter 10 integers to a list and displays the distinct integers separated by exactly one space. Here is a sample run:
Enter ten integers: 34 5 3 5 6 4 33 2 2 4
The distinct integers are 34 5 3 6 4 33 2
11.14
(Combine two lists) Write a method that returns the union of two array lists of
integers using the following header:
public static ArrayList<Integer> union(
ArrayList<Integer> list1, ArrayList<Integer> list2)
448 Chapter 11
Inheritance and Polymorphism
For example, the union of two array lists {2, 3, 1, 5} and {3, 4, 6} is
{2, 3, 1, 5, 3, 4, 6}. Write a test program that prompts the user to enter two lists,
each with five integers, and displays their union. The numbers are separated by
exactly one space in the output. Here is a sample run:
Enter five integers for list1: 3 5 45 4 3
Enter five integers for list2: 33 51 5 4 13
The combined list is 3 5 45 4 3 33 51 5 4 13
*11.15
(Area of a convex polygon) A polygon is convex if it contains any line segments
that connects two points of the polygon. Write a program that prompts the user to
enter the number of points in a convex polygon, then enter the points clockwise,
and display the area of the polygon. Here is a sample run of the program:
Enter the number of the points: 7
Enter the coordinates of the points:
-12 0 -8.5 10 0 11.4 5.5 7.8 6 -5.5 0 -7 -3.5 -3.5
The total area is 250.075
**11.16
(Addition quiz) Rewrite Listing 5.1 RepeatAdditionQuiz.java to alert the user
if an answer is entered again. Hint: use an array list to store answers. Here is a
sample run:
What is 5 + 9? 12
Wrong answer. Try again. What is 5 + 9? 34
Wrong answer. Try again. What is 5 + 9? 12
You already entered 12
Wrong answer. Try again. What is 5 + 9? 14
You got it!
**11.17
(Algebra: perfect square) Write a program that prompts the user to enter an integer m and find the smallest integer n such that m * n is a perfect square. (Hint:
Store all smallest factors of m into an array list. n is the product of the factors that
appear an odd number of times in the array list. For example, consider m = 90,
store the factors 2, 3, 3, 5 in an array list. 2 and 5 appear an odd number of times
in the array list. So, n is 10.) Here are sample runs:
Enter an integer m: 1500
The smallest number n for m * n to be a perfect square is 15
m * n is 22500
Enter an integer m: 63
The smalle
st number n for m * n to be a perfect square is 7
m * n is 441
CHAPTER
EXCEPTION HANDLING
AND TEXT I/O
Objectives
■
To get an overview of exceptions and exception handling (§12.2).
■
To explore the advantages of using exception handling (§12.2).
■
To distinguish exception types: Error (fatal) vs. Exception (nonfatal)
and checked vs. unchecked (§12.3).
■
To declare exceptions in a method header (§12.4.1).
■
To throw exceptions in a method (§12.4.2).
■
To write a try-catch block to handle exceptions (§12.4.3).
■
To explain how an exception is propagated (§12.4.3).
■
To obtain information from an exception object (§12.4.4).
■
To develop applications with exception handling (§12.4.5).
■
To use the finally clause in a try-catch block (§12.5).
■
To use exceptions only for unexpected errors (§12.6).
■
To rethrow exceptions in a catch block (§12.7).
■
To create chained exceptions (§12.8).
■
To define custom exception classes (§12.9).
■
To discover file/directory properties, to delete and rename files/
directories, and to create directories using the File class (§12.10).
■
To write data to a file using the PrintWriter class (§12.11.1).
■
To use try-with-resources to ensure that the resources are closed
automatically (§12.11.2).
■
To read data from a file using the Scanner class (§12.11.3).
■
To understand how data is read using a Scanner (§12.11.4).
■
To develop a program that replaces text in a file (§12.11.5).
■
To read data from the Web (§12.12).
■
To develop a Web crawler (§12.13).
12
450 Chapter 12
Exception Handling and Text I/O
12.1 Introduction
Key
Point
Exception handling enables a program to deal with exceptional situations and
continue its normal execution.
Runtime errors occur while a program is running if the JVM detects an operation that is
impossible to carry out. For example, if you access an array using an index that is out of
bounds, you will get a runtime error with an ArrayIndexOutOfBoundsException. If you
enter a double value when your program expects an integer, you will get a runtime error with
an InputMismatchException.
In Java, runtime errors are thrown as exceptions. An exception is an object that represents
an error or a condition that prevents execution from proceeding normally. If the exception is
not handled, the program will terminate abnormally. How can you handle the exception so
that the program can continue to run or else terminate gracefully? This chapter introduces this
subject and text input and output.
exception
12.2 Exception-Handling Overview
Key
Point
VideoNote
Exception-handling
advantages
read two integers
integer division
Exceptions are thrown from a method. The caller of the method can catch and handle
the exception.
To demonstrate exception handling, including how an exception object is created and thrown,
let’s begin with the example in Listing 12.1, which reads in two integers and displays their
quotient.
LISTING 12.1 Quotient.java
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
import java.util.Scanner;
public class Quotient {
public static void main(String[] args) {
Scanner input = new Scanner(System.in);
// Prompt the user to enter two integers
System.out.print("Enter two integers: ");
int number1 = input.nextInt();
int number2 = input.nextInt();
System.out.println(number1 + " / " + number2 + " is " +
(number1 / number2));
}
}
Enter two integers: 5 2
5 / 2 is 2
Enter two integers: 3 0
Exception in thread "main" java.lang.ArithmeticException: / by zero
at Quotient.main(Quotient.java:11)
If you entered 0 for the second number, a runtime error would occur, because you cannot
divide an integer by 0. (Note that a floating-point number divided by 0 does not raise an
exception.) A simple way to fix this error is to add an if statement to test the second number,
as shown in Listing 12.2.
12.2 Exception-Handling Overview 451
LISTING 12.2 QuotientWithIf.java
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
import java.util.Scanner;
public class QuotientWithIf {
public static void main(String[] args) {
Scanner input = new Scanner(System.in);
// Prompt the user to enter two integers
System.out.print("Enter two integers: ");
int number1 = input.nextInt();
int number2 = input.nextInt();
if (number2 != 0)
System.out.println(number1 + " / " + number2
+ " is " + (number1 / number2));
else
System.out.println("Divisor cannot be zero ");
read two integers
test number2
}
}
Enter two integers: 5 0
Divisor cannot be zero
Before introducing exception handling, let us rewrite Listing 12.2 to compute a quotient using
a method, as shown in Listing 12.3.
LISTING 12.3 QuotientWithMethod.java
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
import java.util.Scanner;
public class QuotientWithMethod {
public static int quotient(int number1, int number2) {
if (number2 == 0) {
System.out.println("Divisor cannot be zero");
System.exit(1);
}
quotient method
terminate the program
return number1 / number2;
}
public static void main(String[] args) {
Scanner input = new Scanner(System.in);
// Prompt the user to enter two integers
System.out.print("Enter two integers: ");
int number1 = input.nextInt();
int number2 = input.nextInt();
int result = quotient(number1, number2);
System.out.println(number1 + " / " + number2 + " is "
+ result);
}
}
read two integers
invoke method
452 Chapter 12
Exception Handling and Text I/O
Enter two integers: 5 3
5 / 3 is 1
Enter two integers: 5 0
Divisor cannot be zero
The method quotient (lines 4–11) returns the quotient of two integers. If number2 is 0, it
cannot return a value, so the program is terminated in line 7. This is clearly a problem. You
should not let the method terminate the program—the caller should decide whether to terminate the program.
How can a method notify its caller an exception has occurred? Java enables a method to
throw an exception that can be caught and handled by the caller. Listing 12.3 can be rewritten,
as shown in Listing 12.4.
LISTING 12.4 QuotientWithException.java
quotient method
throw exception
read two integers
try block
invoke method
catch block
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
import java.util.Scanner;
public class QuotientWithException {
public static int quotient(int number1, int number2) {
if (number2 == 0)
throw new ArithmeticException("Divisor cannot be zero");
return number1 / number2;
}
public static void main(String[] args) {
Scanner input = new Scanner(System.in);
// Prompt the user to enter two integers
System.out.print("Enter two integers: ");
int number1 = input.nextInt();
int number2 = input.nextInt();
try {
int result = quotient(number1, number2);
If an
System.out.println(number1
+ " / " + number2 + " is "
Arithmetic
Exception
+ result);
occurs
}
catch (ArithmeticException ex) {
System.out.println("Exception: an integer " +
"cannot be divided by zero ");
}
System.out.println("Execution continues ...");
}
}
Enter two integers: 5 3
5 / 3 is 1
Execution continues ...
12.2 Exception-Handling Overview 453
Enter two integers: 5 0
Exception: an integer cannot be divided by zero
Execution continues ...
If number2 is 0, the method throws an exception (line 6) by executing
throw new ArithmeticException("Divisor cannot be zero");
The value thrown, in this case new ArithmeticException("Divisor cannot be zero"),
is called an exception. The execution of a throw statement is called throwing an exception.
The exception is an object created from an exception class. In this case, the exception class is
java.lang.ArithmeticException. The constructor ArithmeticException(str) is
invoked to construct an exception object, where str is a message that describes the exception.
When an exception is thrown, the normal execution flow is interrupted. As the name suggests, to “throw an exception” is to pass the exception from one place to another. The statement for invoking the method is contained in a try block and a catch block. The try block
(lines 19–23) contains the code that is executed in normal circumstances. The exception is
caught by the catch block. The code in the catch block is executed to handle the exception.
Afterward, the statement (line 29) after the catch block is executed.
The throw statement is analogous to a method call, but instead of calling a method, it calls
a catch block. In this sense, a catch block is like a method definition with a parameter that
matches the type of the value being thrown. Unlike a method, however, after the catch block
is executed, the program control does not return to the throw statement; instead, it executes
the next statement after the catch block.
The identifier ex in the catch–block header
throw statement
exception
throw exception
handle exception
catch (ArithmeticException ex)
acts very much like a parameter in a method. Thus, this parameter is referred to as a
catch–block parameter. The type (e.g., ArithmeticException) preceding ex specifies
what kind of exception the catch block can catch. Once the exception is caught, you can
access the thrown value from this parameter in the body of a catch block.
In summary, a template for a try-throw-catch block may look like this:
catch–block parameter
try {
Code to run;
A statement or a method that may throw an exception;
More code to run;
}
catch (type ex) {
Code to process the exception;
}
An exception may be thrown directly by using a throw statement in a try block, or by invoking a method that may throw an exception.
The main method invokes quotient (line 20). If the quotient method executes normally,
it returns a value to the caller. If the quotient method encounters an exception, it throws the
exception back to its caller. The caller’s catch block handles the exception.
Now you can see the advantage of using exception handling: It enables a method to throw
an exception to its caller, enabling the caller to handle the exception. Without this capability,
the called method itself must handle the exception or terminate the program. Often the called
method does not know what to do in case of error. This is typically the case for the library
methods. The library method can detect the error, but only the caller knows what needs to be
advantage
454 Chapter 12
Exception Handling and Text I/O
done when an error occurs. The key benefit of exception handling is separating the detection
of an error (done in a called method) from the handling of an error (done in the calling
method).
Many library methods throw exceptions. Listing 12.5 gives an example that handles an
InputMismatchException when reading an input.
LISTING 12.5 InputMismatchExceptionDemo.java
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
create a Scanner
try block
catch block
import java.util.*;
public class InputMismatchExceptionDemo {
public static void main(String[] args) {
Scanner input = new Scanner(System.in);
boolean continueInput = true;
do {
try {
System.out.print("Enter an integer: ");
int number = input.nextInt();
If an
InputMismatch
Exception
occurs
// Display the result
System.out.println(
"The number entered is " + number);
continueInput = false;
}
catch (InputMismatchException ex) {
System.out.println("Try again. (" +
"Incorrect input: an integer is required)");
input.nextLine(); // Discard input
}
} while (continueInput);
}
}
Enter an integer: 3.5
Try again. (Incorrect input: an integer is required)
Enter an integer: 4
The number entered is 4
When executing input.nextInt() (line 11), an InputMismatchException occurs if
the input entered is not an integer. Suppose 3.5 is entered. An InputMismatchException
occurs and the control is transferred to the catch block. The statements in the catch block
are now executed. The statement input.nextLine() in line 22 discards the current input
line so that the user can enter a new line of input. The variable continueInput controls the
loop. Its initial value is true (line 6), and it is changed to false (line 17) when a valid input
is received. Once a valid input is received, there is no need to continue the input.
✓
Check
Point
12.1 What is the advantage of using exception handling?
12.2 Which of the following statements will throw an exception?
System.out.println(1 / 0);
System.out.println(1.0 / 0);
12.3 Exception Types 455
12.3 Point out the problem in the following code. Does the code throw any exceptions?
long value = Long.MAX_VALUE + 1;
System.out.println(value);
12.4 What does the JVM do when an exception occurs? How do you catch an exception?
12.5 What is the output of the following code?
public class Test {
public static void main(String[] args) {
try {
int value = 30;
if (value < 40)
throw new Exception("value is too small");
}
catch (Exception ex) {
System.out.println(ex.getMessage());
}
System.out.println("Continue after the catch block");
}
}
What would be the output if the line
int value = 30;
were changed to
int value = 50;
12.6 Show the output of the following code.
public class Test {
public static void main(String[] args) {
for (int i = 0; i < 2; i++) {
System.out.print(i + " ");
try {
System.out.println(1 / 0);
}
catch (Exception ex) {
}
}
}
}
public class Test {
public static void main(String[] args) {
try {
for (int i = 0; i < 2; i++) {
System.out.print(i + " ");
System.out.println(1 / 0);
}
}
catch (Exception ex) {
}
}
}
(a)
(b)
12.3 Exception Types
Exceptions are objects, and objects are defined using classes. The root class for
exceptions is java.lang.Throwable.
The preceding section used the classes ArithmeticException and InputMismatchException. Are there any other types of exceptions you can use? Can you define your
own exception classes? Yes. There are many predefined exception classes in the Java API.
Figure 12.1 shows some of them, and in Section 12.9 you will learn how to define your own
exception classes.
Key
Point
456 Chapter 12
Exception Handling and Text I/O
ClassNotFoundException
ArithmeticException
IOException
Exception
NullPointerException
RuntimeException
IndexOutOfBoundsException
Many more classes
Object
Throwable
IllegalArgumentException
LinkageError
Many more classes
Error
VirtualMachineError
Many more classes
FIGURE 12.1 Exceptions thrown are instances of the classes shown in this diagram, or of subclasses of one of these classes.
Note
The class names Error, Exception, and RuntimeException are somewhat confusing. All three of these classes are exceptions, and all of the errors occur at runtime.
The Throwable class is the root of exception classes. All Java exception classes inherit
directly or indirectly from Throwable. You can create your own exception classes by extending Exception or a subclass of Exception.
The exception classes can be classified into three major types: system errors, exceptions,
and runtime exceptions.
■
system error
System errors are thrown by the JVM and are represented in the Error class. The
Error class describes internal system errors, though such errors rarely occur. If one
does, there is little you can do beyond notifying the user and trying to terminate the
program gracefully. Examples of subclasses of Error are listed in Table 12.1.
TABLE 12.1 Examples of Subclasses of Error
exception
Class
Reasons for Exception
LinkageError
A class has some dependency on another class, but the latter class has
changed incompatibly after the compilation of the former class.
VirtualMachineError
The JVM is broken or has run out of the resources it needs in order to
continue operating.
■
Exceptions are represented in the Exception class, which describes errors caused by
your program and by external circumstances. These errors can be caught and handled
by your program. Examples of subclasses of Exception are listed in Table 12.2.
TABLE 12.2 Examples of Subclasses of Exception
Class
Reasons for Exception
ClassNotFoundException
Attempt to use a class that does not exist. This exception would occur, for example, if you tried to
run a nonexistent class using the java command, or if your program were composed of, say,
three class files, only two of which could be found.
IOException
Related to input/output operations, such as invalid input, reading past the end of a file, and opening
a nonexistent file. Examples of subclasses of IOException are InterruptedIOException,
EOFException (EOF is short for End of File), and FileNotFoundException.
12.3 Exception Types 457
■
Runtime exceptions are represented in the RuntimeException class, which
describes programming errors, such as bad casting, accessing an out-of-bounds
array, and numeric errors. Runtime exceptions are generally thrown by the JVM.
Examples of subclasses are listed in Table 12.3.
runtime exception
TABLE 12.3 Examples of Subclasses of RuntimeException
Class
Reasons for Exception
ArithmeticException
Dividing an integer by zero. Note that floating-point arithmetic
does not throw exceptions (see Appendix E, Special FloatingPoint Values).
NullPointerException
Attempt to access an object through a null reference variable.
IndexOutOfBoundsException
Index to an array is out of range.
IllegalArgumentException
A method is passed an argument that is illegal or inappropriate.
RuntimeException, Error, and their subclasses are known as unchecked exceptions. All
other exceptions are known as checked exceptions, meaning that the compiler forces the
programmer to check and deal with them in a try-catch block or declare it in the method
header. Declaring an exception in the method header will be covered in Section 12.4.
In most cases, unchecked exceptions reflect programming logic errors that are unrecoverable. For example, a NullPointerException is thrown if you access an object through a
reference variable before an object is assigned to it; an IndexOutOfBoundsException is
thrown if you access an element in an array outside the bounds of the array. These are logic
errors that should be corrected in the program. Unchecked exceptions can occur anywhere in
a program. To avoid cumbersome overuse of try-catch blocks, Java does not mandate that
you write code to catch or declare unchecked exceptions.
12.7 Describe the Java Throwable class, its subclasses, and the types of exceptions.
12.8 What RuntimeException will the following programs throw, if any?
unchecked exception
checked exception
✓
Check
Point
public class Test {
public static void main(String[] args) {
System.out.println(1 / 0);
}
}
public class Test {
public static void main(String[] args) {
int[] list = new int[5];
System.out.println(list[5]);
}
}
(a)
(b)
public class Test {
public static void main(String[] args) {
String s = "abc";
System.out.println(s.charAt(3));
}
}
public class Test {
public static void main(String[] args) {
Object o = new Object();
String d = (String)o;
}
}
(c)
(d)
public class Test {
public static void main(String[] args) {
Object o = null;
System.out.println(o.toString());
}
}
public class Test {
public static void main(String[] args) {
System.out.println(1.0 / 0);
}
}
(e)
(f)
458 Chapter 12
Exception Handling and Text I/O
12.4 More on Exception Handling
Key
Point
A handler for an exception is found by propagating the exception backward through a
chain of method calls, starting from the current method.
The preceding sections gave you an overview of exception handling and introduced several predefined exception types. This section provides an in-depth discussion of exception
handling.
Java’s exception-handling model is based on three operations: declaring an exception,
throwing an exception, and catching an exception, as shown in Figure 12.2.
method1() {
method2() throws Exception {
try {
invoke method2;
}
catch (Exception ex) {
Process exception;
}
Catch exception
Declare exception
if (an error occurs) {
throw new Exception();
Throw exception
}
}
}
FIGURE 12.2 Exception handling in Java consists of declaring exceptions, throwing exceptions, and catching and
processing exceptions.
12.4.1
declare exception
Declaring Exceptions
In Java, the statement currently being executed belongs to a method. The Java interpreter
invokes the main method to start executing a program. Every method must state the types of
checked exceptions it might throw. This is known as declaring exceptions. Because system
errors and runtime errors can happen to any code, Java does not require that you declare
Error and RuntimeException (unchecked exceptions) explicitly in the method. However,
all other exceptions thrown by the method must be explicitly declared in the method header so
that the caller of the method is informed of the exception.
To declare an exception in a method, use the throws keyword in the method header, as in
this example:
public void myMethod() throws IOException
The throws keyword indicates that myMethod might throw an IOException. If the method
might throw multiple exceptions, add a list of the exceptions, separated by commas, after
throws:
public void myMethod()
throws Exception1, Exception2, ..., ExceptionN
Note
If a method does not declare exceptions in the superclass, you cannot override it to
declare exceptions in the subclass.
12.4.2
throw exception
Throwing Exceptions
A program that detects an error can create an instance of an appropriate exception type and
throw it. This is known as throwing an exception. Here is an example: Suppose the program
detects that an argument passed to the method violates the method contract (e.g., the argument
12.4 More on Exception Handling 459
must be nonnegative, but a negative argument is passed); the program can create an instance
of IllegalArgumentException and throw it, as follows:
IllegalArgumentException ex =
new IllegalArgumentException("Wrong Argument");
throw ex;
Or, if you prefer, you can use the following:
throw new IllegalArgumentException("Wrong Argument");
Note
IllegalArgumentException is an exception class in the Java API. In general,
each exception class in the Java API has at least two constructors: a no-arg constructor, and a constructor with a String argument that describes the exception.
This argument is called the exception message, which can be obtained using
getMessage().
exception message
Tip
The keyword to declare an exception is throws, and the keyword to throw an exception
is throw.
12.4.3
throws vs. throw
Catching Exceptions
You now know how to declare an exception and how to throw an exception. When an exception is thrown, it can be caught and handled in a try-catch block, as follows:
catch exception
try {
statements; // Statements that may throw exceptions
}
catch (Exception1 exVar1) {
handler for exception1;
}
catch (Exception2 exVar2) {
handler for exception2;
}
...
catch (ExceptionN exVarN) {
handler for exceptionN;
}
If no exceptions arise during the execution of the try block, the catch blocks are skipped.
If one of the statements inside the try block throws an exception, Java skips the remaining statements in the try block and starts the process of finding the code to handle the
exception. The code that handles the exception is called the exception handler; it is found
by propagating the exception backward through a chain of method calls, starting from the
current method. Each catch block is examined in turn, from first to last, to see whether
the type of the exception object is an instance of the exception class in the catch block.
If so, the exception object is assigned to the variable declared, and the code in the catch
block is executed. If no handler is found, Java exits this method, passes the exception to
the method that invoked the method, and continues the same process to find a handler. If
no handler is found in the chain of methods being invoked, the program terminates and
prints an error message on the console. The process of finding a handler is called catching
an exception.
exception handler
exception propagation
460 Chapter 12
Exception Handling and Text I/O
Suppose the main method invokes method1, method1 invokes method2, method2
invokes method3, and method3 throws an exception, as shown in Figure 12.3. Consider the
following scenario:
main method {
...
try {
...
invoke method1;
statement1;
}
catch (Exception1 ex1) {
Process ex1;
}
statement2;
}
■
If the exception type is Exception3, it is caught by the catch block for handling exception ex3 in method2. statement5 is skipped, and statement6 is
executed.
■
If the exception type is Exception2, method2 is aborted, the control is returned to
method1, and the exception is caught by the catch block for handling exception
ex2 in method1. statement3 is skipped, and statement4 is executed.
■
If the exception type is Exception1, method1 is aborted, the control is returned
to the main method, and the exception is caught by the catch block for handling
exception ex1 in the main method. statement1 is skipped, and statement2 is
executed.
■
If the exception type is not caught in method2, method1, or main, the program
terminates, and statement1 and statement2 are not executed.
method1 {
...
try {
...
invoke method2;
statement3;
}
catch (Exception2 ex2) {
Process ex2;
}
statement4;
}
method2 {
...
try {
...
invoke method3;
statement5;
}
catch (Exception3 ex3) {
Process ex3;
}
statement6;
}
An exception
is thrown in
method3
Call stack
method3
main method
method2
method2
method1
method1
method1
main method
main method
main method
FIGURE 12.3 If an exception is not caught in the current method, it is passed to its caller. The process is repeated until
the exception is caught or passed to the main method.
Note
catch block
Various exception classes can be derived from a common superclass. If a catch block
catches exception objects of a superclass, it can catch all the exception objects of the
subclasses of that superclass.
Note
order of exception handlers
The order in which exceptions are specified in catch blocks is important. A compile
error will result if a catch block for a superclass type appears before a catch block for a
subclass type. For example, the ordering in (a) on the next page is erroneous, because
RuntimeException is a subclass of Exception. The correct ordering should be as
shown in (b).
12.4 More on Exception Handling 461
try {
...
}
catch (Exception ex) {
...
}
catch (RuntimeException ex) {
...
}
try {
...
}
catch (RuntimeException ex) {
...
}
catch (Exception ex) {
...
}
(a) Wrong order
(b) Correct order
Note
Java forces you to deal with checked exceptions. If a method declares a checked exception
(i.e., an exception other than Error or RuntimeException), you must invoke it in a
try-catch block or declare to throw the exception in the calling method. For example,
suppose that method p1 invokes method p2, and p2 may throw a checked exception
(e.g., IOException); you have to write the code as shown in (a) or (b) below.
void p1() {
try {
p2();
}
catch (IOException ex) {
...
}
}
(a) Catch exception
catch or declare checked
exceptions
void p1() throws IOException {
p2();
}
(b) Throw exception
Note
You can use the new JDK 7 multi-catch feature to simplify coding for the exceptions
with the same handling code. The syntax is:
JDK 7 multi-catch
catch (Exception1 | Exception2 | ... | Exceptionk ex) {
// Same code for handling these exceptions
}
Each exception type is separated from the next with a vertical bar (|). If one of the
exceptions is caught, the handling code is executed.
12.4.4
Getting Information from Exceptions
An exception object contains valuable information about the exception. You may use the following instance methods in the java.lang.Throwable class to get information regarding
the exception, as shown in Figure 12.4. The printStackTrace() method prints stack trace
methods in Throwable
java.lang.Throwable
+getMessage(): String
+toString(): String
Returns the message that describes this exception object.
Returns the concatenation of three strings: (1) the full name of the exception
class; (2) ":" (a colon and a space); (3) the getMessage() method.
+printStackTrace(): void
Prints the Throwable object and its call stack trace information on the
console.
+getStackTrace():
StackTraceElement[]
Returns an array of stack trace elements representing the stack trace
pertaining to this exception object.
FIGURE 12.4 Throwable is the root class for all exception objects.
462 Chapter 12
Exception Handling and Text I/O
information on the console. The getStackTrace() method provides programmatic access
to the stack trace information printed by printStackTrace().
Listing 12.6 gives an example that uses the methods in Throwable to display exception
information. Line 4 invokes the sum method to return the sum of all the elements in the array.
There is an error in line 23 that causes the ArrayIndexOutOfBoundsException, a subclass of IndexOutOfBoundsException. This exception is caught in the try-catch block.
Lines 7, 8, and 9 display the stack trace, exception message, and exception object and message using the printStackTrace(), getMessage(), and toString() methods, as shown
in Figure 12.5. Line 12 brings stack trace elements into an array. Each element represents a
method call. You can obtain the method (line 14), class name (line 15), and exception line
number (line 16) for each element.
printStackTrace()
getMessage()
toString()
Using
getStackTrace()
FIGURE 12.5
You can use the printStackTrace(), getMessage(), toString(), and
getStackTrace() methods to obtain information from exception objects.
LISTING 12.6 TestException.java
invoke sum
printStackTrace()
getMessage()
toString()
getStackTrace()
cause an exception
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
public class TestException {
public static void main(String[] args) {
try {
System.out.println(sum(new int[] {1, 2, 3, 4, 5}));
}
catch (Exception ex) {
ex.printStackTrace();
System.out.println("\n" + ex.getMessage());
System.out.println("\n" + ex.toString());
System.out.println("\nTrace Info Obtained from getStackTrace");
StackTraceElement[] traceElements = ex.getStackTrace();
for (int i = 0; i < traceElements.length; i++) {
System.out.print("method " + traceElements[i].getMethodName());
System.out.print("(" + traceElements[i].getClassName() + ":");
System.out.println(traceElements[i].getLineNumber() + ")");
}
}
}
private static int sum(int[] list) {
int result = 0;
for (int i = 0; i <= list.length; i++)
12.4 More on Exception Handling 463
24
25
26
27
result += list[i];
return result;
}
}
12.4.5
Example: Declaring, Throwing, and Catching Exceptions
This example demonstrates declaring, throwing, and catching exceptions by modifying the
setRadius method in the Circle class in Listing 9.8, CircleWithPrivateDataFields.java.
The new setRadius method throws an exception if the radius is negative.
Listing 12.7 defines a new circle class named CircleWithException, which is the same
as CircleWithPrivateDataFields except that the setRadius(double newRadius)
method throws an IllegalArgumentException if the argument newRadius is negative.
LISTING 12.7 CircleWithException.java
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
public class CircleWithException {
/** The radius of the circle */
private double radius;
/** The number of the objects created */
private static int numberOfObjects = 0;
/** Construct a circle with radius 1 */
public CircleWithException() {
this(1.0);
}
/** Construct a circle with a specified radius */
public CircleWithException(double newRadius) {
setRadius(newRadius);
numberOfObjects++;
}
/** Return radius */
public double getRadius() {
return radius;
}
/** Set a new radius */
public void setRadius(double newRadius)
throws IllegalArgumentException {
if (newRadius >= 0)
radius = newRadius;
else
throw new IllegalArgumentException(
"Radius cannot be negative");
}
/** Return numberOfObjects */
public static int getNumberOfObjects() {
return numberOfObjects;
}
/** Return the area of this circle */
public double findArea() {
return radius * radius * 3.14159;
}
}
declare exception
throw exception
464 Chapter 12
Exception Handling and Text I/O
A test program that uses the new Circle class is given in Listing 12.8.
LISTING 12.8 TestCircleWithException.java
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
try
catch
public class TestCircleWithException {
public static void main(String[] args) {
try {
CircleWithException c1 = new CircleWithException(5);
CircleWithException c2 = new CircleWithException(-5);
CircleWithException c3 = new CircleWithException(0);
}
catch (IllegalArgumentException ex) {
System.out.println(ex);
}
System.out.println("Number of objects created: " +
CircleWithException.getNumberOfObjects());
}
}
java.lang.IllegalArgumentException: Radius cannot be negative
Number of objects created: 1
The original Circle class remains intact except that the class name is changed to
CircleWithException, a new constructor CircleWithException(newRadius) is
added, and the setRadius method now declares an exception and throws it if the radius is
negative.
The setRadius method declares to throw IllegalArgumentException in the method
header (lines 25–32 in CircleWithException.java). The CircleWithException class would
still compile if the throws IllegalArgumentException clause (line 26) were removed from
the method declaration, since it is a subclass of RuntimeException and every method can
throw RuntimeException (an unchecked exception) regardless of whether it is declared in the
method header.
The test program creates three CircleWithException objects—c1, c2, and c3—to test
how to handle exceptions. Invoking new CircleWithException(-5) (line 5 in Listing 12.8)
causes the setRadius method to be invoked, which throws an IllegalArgumentException,
because the radius is negative. In the catch block, the type of the object ex is
IllegalArgumentException, which matches the exception object thrown by the setRadius
method, so this exception is caught by the catch block.
The exception handler prints a short message, ex.toString() (line 9 in Listing 12.8),
about the exception, using System.out.println(ex).
Note that the execution continues in the event of the exception. If the handlers had not
caught the exception, the program would have abruptly terminated.
The test program would still compile if the try statement were not used, because the method
throws an instance of IllegalArgumentException, a subclass of RuntimeException (an
unchecked exception). If a method throws an exception other than RuntimeException or
Error, the method must be invoked within a try-catch block.
✓
Check
Point
12.9 What is the purpose of declaring exceptions? How do you declare an exception, and
where? Can you declare multiple exceptions in a method header?
12.10 What is a checked exception, and what is an unchecked exception?
12.11 How do you throw an exception? Can you throw multiple exceptions in one throw
statement?
12.12 What is the keyword throw used for? What is the keyword throws used for?
12.4 More on Exception Handling 465
12.13 Suppose that statement2 causes an exception in the following try-catch block:
try {
statement1;
statement2;
statement3;
}
catch (Exception1 ex1) {
}
catch (Exception2 ex2) {
}
statement4;
Answer the following questions:
■
■
Will statement3 be executed?
If the exception is not caught, will statement4 be executed?
■
If the exception is caught in the catch block, will statement4 be executed?
12.14 What is displayed when the following program is run?
public class Test {
public static void main(String[] args) {
try {
int[] list = new int[10];
System.out.println("list[10] is " + list[10]);
}
catch (ArithmeticException ex) {
System.out.println("ArithmeticException");
}
catch (RuntimeException ex) {
System.out.println("RuntimeException");
}
catch (Exception ex) {
System.out.println("Exception");
}
}
}
12.15 What is displayed when the following program is run?
public class Test {
public static void main(String[] args) {
try {
method();
System.out.println("After the method call");
}
catch (ArithmeticException ex) {
System.out.println("ArithmeticException");
}
catch (RuntimeException ex) {
System.out.println("RuntimeException");
}
catch (Exception e) {
System.out.println("Exception");
}
}
static void method() throws Exception {
466 Chapter 12
Exception Handling and Text I/O
System.out.println(1 / 0);
}
}
12.16 What is displayed when the following program is run?
public class Test {
public static void main(String[] args) {
try {
method();
System.out.println("After the method call");
}
catch (RuntimeException ex) {
System.out.println("RuntimeException in main");
}
catch (Exception ex) {
System.out.println("Exception in main");
}
}
static void method() throws Exception {
try {
String s = "abc";
System.out.println(s.charAt(3));
}
catch (RuntimeException ex) {
System.out.println("RuntimeException in method()");
}
catch (Exception ex) {
System.out.println("Exception in method()");
}
}
}
12.17
12.18
12.19
12.20
What does the method getMessage() do?
What does the method printStackTrace() do?
Does the presence of a try-catch block impose overhead when no exception occurs?
Correct a compile error in the following code:
public void m(int value) {
if (value < 40)
throw new Exception("value is too small");
}
12.5 The finally Clause
Key
Point
The finally clause is always executed regardless whether an exception occurred or not.
Occasionally, you may want some code to be executed regardless of whether an exception
occurs or is caught. Java has a finally clause that can be used to accomplish this objective.
The syntax for the finally clause might look like this:
try {
statements;
}
catch (TheException ex) {
handling ex;
}
12.6 When to Use Exceptions 467
finally {
finalStatements;
}
The code in the finally block is executed under all circumstances, regardless of whether an
exception occurs in the try block or is caught. Consider three possible cases:
■
If no exception arises in the try block, finalStatements is executed, and the next
statement after the try statement is executed.
■
If a statement causes an exception in the try block that is caught in a catch block, the
rest of the statements in the try block are skipped, the catch block is executed, and the
finally clause is executed. The next statement after the try statement is executed.
■
If one of the statements causes an exception that is not caught in any catch block,
the other statements in the try block are skipped, the finally clause is executed,
and the exception is passed to the caller of this method.
The finally block executes even if there is a return statement prior to reaching the
finally block.
Note
The catch block may be omitted when the finally clause is used.
12.21 Suppose that statement2 causes an exception in the following statement:
try {
statement1;
statement2;
statement3;
}
catch (Exception1 ex1) {
}
finally {
statement4;
}
statement5;
omit catch block
✓
Check
Point
Answer the following questions:
■
If no exception occurs, will statement4 be executed, and will statement5 be
executed?
■
If the exception is of type Exception1, will statement4 be executed, and will
statement5 be executed?
■
If the exception is not of type Exception1, will statement4 be executed, and
will statement5 be executed?
12.6 When to Use Exceptions
A method should throw an exception if the error needs to be handled by its caller.
The try block contains the code that is executed in normal circumstances. The catch block
contains the code that is executed in exceptional circumstances. Exception handling separates
error-handling code from normal programming tasks, thus making programs easier to read
and to modify. Be aware, however, that exception handling usually requires more time and
resources, because it requires instantiating a new exception object, rolling back the call stack,
and propagating the exception through the chain of methods invoked to search for the handler.
Key
Point
468 Chapter 12
Exception Handling and Text I/O
An exception occurs in a method. If you want the exception to be processed by its caller,
you should create an exception object and throw it. If you can handle the exception in the
method where it occurs, there is no need to throw or use exceptions.
In general, common exceptions that may occur in multiple classes in a project are candidates for exception classes. Simple errors that may occur in individual methods are best handled without throwing exceptions. This can be done by using if statements to check for errors.
When should you use a try-catch block in the code? Use it when you have to deal with
unexpected error conditions. Do not use a try-catch block to deal with simple, expected
situations. For example, the following code
try {
System.out.println(refVar.toString());
}
catch (NullPointerException ex) {
System.out.println("refVar is null");
}
is better replaced by
if (refVar != null)
System.out.println(refVar.toString());
else
System.out.println("refVar is null");
Which situations are exceptional and which are expected is sometimes difficult to decide. The
point is not to abuse exception handling as a way to deal with a simple logic test.
✓
Check
Point
12.22 The following method checks whether a string is a numeric string:
public static boolean isNumeric(String token) {
try {
Double.parseDouble(token);
return true;
}
catch (java.lang.NumberFormatException ex) {
return false;
}
}
Is it correct? Rewrite it without using exceptions.
12.7 Rethrowing Exceptions
Key
Point
Java allows an exception handler to rethrow the exception if the handler cannot
process the exception or simply wants to let its caller be notified of the exception.
The syntax for rethrowing an exception may look like this:
try {
statements;
}
catch (TheException ex) {
perform operations before exits;
throw ex;
}
The statement throw ex rethrows the exception to the caller so that other handlers in the
caller get a chance to process the exception ex.
12.8 Chained Exceptions 469
12.23 Suppose that statement2 causes an exception in the following statement:
try {
statement1;
statement2;
statement3;
}
catch (Exception1 ex1) {
}
catch (Exception2 ex2) {
throw ex2;
}
finally {
statement4;
}
statement5;
✓
Check
Point
Answer the following questions:
■
If no exception occurs, will statement4 be executed, and will statement5 be
executed?
■
If the exception is of type Exception1, will statement4 be executed, and will
statement5 be executed?
■
If the exception is of type Exception2, will statement4 be executed, and will
statement5 be executed?
■
If the exception is not Exception1 nor Exception2, will statement4 be executed, and will statement5 be executed?
12.8 Chained Exceptions
Throwing an exception along with another exception forms a chained exception.
In the preceding section, the catch block rethrows the original exception. Sometimes, you
may need to throw a new exception (with additional information) along with the original
exception. This is called chained exceptions. Listing 12.9 illustrates how to create and throw
chained exceptions.
Key
Point
chained exception
LISTING 12.9 ChainedExceptionDemo.java
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
public class ChainedExceptionDemo {
public static void main(String[] args) {
try {
method1();
}
catch (Exception ex) {
ex.printStackTrace();
}
}
public static void method1() throws Exception {
try {
method2();
}
catch (Exception ex) {
throw new Exception("New info from method1", ex);
}
}
stack trace
chained exception
470 Chapter 12
Exception Handling and Text I/O
19
20
21
22
23
throw exception
public static void method2() throws Exception {
throw new Exception("New info from method2");
}
}
java.lang.Exception: New info from method1
at ChainedExceptionDemo.method1(ChainedExceptionDemo.java:16)
at ChainedExceptionDemo.main(ChainedExceptionDemo.java:4)
Caused by: java.lang.Exception: New info from method2
at ChainedExceptionDemo.method2(ChainedExceptionDemo.java:21)
at ChainedExceptionDemo.method1(ChainedExceptionDemo.java:13)
... 1 more
The main method invokes method1 (line 4), method1 invokes method2 (line 13), and
method2 throws an exception (line 21). This exception is caught in the catch block in
method1 and is wrapped in a new exception in line 16. The new exception is thrown and
caught in the catch block in the main method in line 6. The sample output shows the output
from the printStackTrace() method in line 7. The new exception thrown from method1
is displayed first, followed by the original exception thrown from method2.
✓
Check
Point
12.24 What would be the output if line 16 is replaced by the following line?
throw new Exception(“New info from method1”);
12.9 Defining Custom Exception Classes
Key
Point
VideoNote
Create custom exception
classes
You can define a custom exception class by extending the java.lang.Exception
class.
Java provides quite a few exception classes. Use them whenever possible instead of defining
your own exception classes. However, if you run into a problem that cannot be adequately
described by the predefined exception classes, you can create your own exception class,
derived from Exception or from a subclass of Exception, such as IOException.
In Listing 12.7, CircleWithException.java, the setRadius method throws an exception if
the radius is negative. Suppose you wish to pass the radius to the handler. In that case, you can
define a custom exception class, as shown in Listing 12.10.
LISTING 12.10 InvalidRadiusException.java
extends Exception
1
2
3
4
5
6
7
8
9
10
11
12
13
14
public class InvalidRadiusException extends Exception {
private double radius;
/** Construct an exception */
public InvalidRadiusException(double radius) {
super("Invalid radius " + radius);
this.radius = radius;
}
/** Return the radius */
public double getRadius() {
return radius;
}
}
This custom exception class extends java.lang.Exception (line 1). The Exception class
extends java.lang.Throwable. All the methods (e.g., getMessage(), toString(), and
12.9 Defining Custom Exception Classes 471
printStackTrace()) in Exception are inherited from Throwable. The Exception
class contains four constructors. Among them, the following two constructors are often used:
java.lang.Exception
+Exception()
Constructs an exception with no message.
+Exception(message: String)
Constructs an exception with the specified message.
Line 6 invokes the superclass’s constructor with a message. This message will be set in the
exception object and can be obtained by invoking getMessage() on the object.
Tip
Most exception classes in the Java API contain two constructors: a no-arg constructor
and a constructor with a message parameter.
To create an InvalidRadiusException, you have to pass a radius. Therefore, the setRadius method in Listing 12.7 can be modified as shown in Listing 12.11.
LISTING 12.11 TestCircleWithCustomException.java
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
public class TestCircleWithCustomException {
public static void main(String[] args) {
try {
new CircleWithCustomException(5);
new CircleWithCustomException(-5);
new CircleWithCustomException(0);
}
catch (InvalidRadiusException ex) {
System.out.println(ex);
}
System.out.println("Number of objects created: " +
CircleWithCustomException.getNumberOfObjects());
}
}
class CircleWithCustomException {
/** The radius of the circle */
private double radius;
/** The number of objects created */
private static int numberOfObjects = 0;
/** Construct a circle with radius 1 */
public CircleWithCustomException() throws InvalidRadiusException {
this(1.0);
}
/** Construct a circle with a specified radius */
public CircleWithCustomException(double newRadius)
throws InvalidRadiusException {
setRadius(newRadius);
numberOfObjects++;
}
/** Return radius */
public double getRadius() {
declare exception
472 Chapter 12
Exception Handling and Text I/O
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
throw exception
return radius;
}
/** Set a new radius */
public void setRadius(double newRadius)
throws InvalidRadiusException {
if (newRadius >= 0)
radius = newRadius;
else
throw new InvalidRadiusException(newRadius);
}
/** Return numberOfObjects */
public static int getNumberOfObjects() {
return numberOfObjects;
}
/** Return the area of this circle */
public double findArea() {
return radius * radius * 3.14159;
}
}
InvalidRadiusException: Invalid radius -5.0
Number of objects created: 1
The setRadius method in CircleWithCustomException throws an InvalidRadiusException when radius is negative (line 47). Since InvalidRadiusException is a
checked exception, the setRadius method must declare it in the method header (line 43).
Since the constructors for CircleWithCustomException invoke the setRadius method
to a set a new radius and it may throw an InvalidRadiusException, the constructors are
declared to throw InvalidRadiusException (lines 25, 31).
Invoking new CircleWithCustomException(-5) (line 5) throws an InvalidRadiusException, which is caught by the handler. The handler displays the radius in the exception
object ex.
Tip
checked custom exception
✓
Check
Point
Can you define a custom exception class by extending RuntimeException? Yes, but
it is not a good way to go, because it makes your custom exception unchecked. It is better to make a custom exception checked, so that the compiler can force these exceptions
to be caught in your program.
12.25 How do you define a custom exception class?
12.26 Suppose the setRadius method throws the InValidRadiusException defined in
Listing 12.10. What is displayed when the following program is run?
public class Test {
public static void main(String[] args) {
try {
method();
System.out.println("After the method call");
}
catch (RuntimeException ex) {
System.out.println("RuntimeException in main");
}
12.10 The File Class 473
catch (Exception ex) {
System.out.println("Exception in main");
}
}
static void method() throws Exception {
try {
Circle c1 = new Circle(1);
c1.setRadius(-1);
System.out.println(c1.getRadius());
}
catch (RuntimeException ex) {
System.out.println("RuntimeException in method()");
}
catch (Exception ex) {
System.out.println("Exception in method()");
throw ex;
}
}
}
12.10 The File Class
The File class contains the methods for obtaining the properties of a file/directory
and for renaming and deleting a file/directory.
Having learned exception handling, you are ready to step into file processing. Data stored in
the program are temporary; they are lost when the program terminates. To permanently store
the data created in a program, you need to save them in a file on a disk or other permanent
storage device. The file can then be transported and read later by other programs. Since data
are stored in files, this section introduces how to use the File class to obtain file/directory
properties, to delete and rename files/directories, and to create directories. The next section
introduces how to read/write data from/to text files.
Every file is placed in a directory in the file system. An absolute file name (or full
name) contains a file name with its complete path and drive letter. For example, c:\book\
Welcome.java is the absolute file name for the file Welcome.java on the Windows
operating system. Here c:\book is referred to as the directory path for the file. Absolute
file names are machine dependent. On the UNIX platform, the absolute file name may
be /home/liang/book/Welcome.java, where /home/liang/book is the directory path for
the file Welcome.java.
A relative file name is in relation to the current working directory. The complete
directory path for a relative file name is omitted. For example, Welcome.java is a relative file name. If the current working directory is c:\book, the absolute file name would
be c:\book\Welcome.java.
The File class is intended to provide an abstraction that deals with most of the machinedependent complexities of files and path names in a machine-independent fashion. The File
class contains the methods for obtaining file and directory properties and for renaming and
deleting files and directories, as shown in Figure 12.6. However, the File class does not
contain the methods for reading and writing file contents.
The file name is a string. The File class is a wrapper class for the file name and its directory path. For example, new File("c:\\book") creates a File object for the directory
c:\book, and new File("c:\\book\\test.dat") creates a File object for the
file c:\book\test.dat, both on Windows. You can use the File class’s isDirectory()
method to check whether the object represents a directory, and the isFile() method to
check whether the object represents a file.
Key
Point
why file?
absolute file name
directory path
relative file name
474 Chapter 12
Exception Handling and Text I/O
java.io.File
+File(pathname: String)
+File(parent: String, child: String)
Creates a File object for the specified path name. The path name may be a
directory or a file.
Creates a File object for the child under the directory parent. The child may be
a file name or a subdirectory.
+File(parent: File, child: String)
Creates a File object for the child under the directory parent. The parent is a
File object. In the preceding constructor, the parent is a string.
+exists(): boolean
Returns true if the file or the directory represented by the File object exists.
+canRead(): boolean
Returns true if the file represented by the File object exists and can be read.
+canWrite(): boolean
Returns true if the file represented by the File object exists and can be written.
+isDirectory(): boolean
Returns true if the File object represents a directory.
+isFile(): boolean
Returns true if the File object represents a file.
+isAbsolute(): boolean
Returns true if the File object is created using an absolute path name.
+isHidden(): boolean
Returns true if the file represented in the File object is hidden. The exact
definition of hidden is system-dependent. On Windows, you can mark a file
hidden in the File Properties dialog box. On Unix systems, a file is hidden if
its name begins with a period(.) character.
+getAbsolutePath(): String
Returns the complete absolute file or directory name represented by the File
object.
+getCanonicalPath(): String
Returns the same as getAbsolutePath() except that it removes redundant
names, such as "." and "..", from the path name, resolves symbolic links (on
Unix), and converts drive letters to standard uppercase (on Windows).
+getName(): String
Returns the last name of the complete directory and file name represented by
the File object. For example, new File("c:\\book\\test.dat").getName() returns
test.dat.
Returns the complete directory and file name represented by the File object.
For example, new File("c:\\book\\test.dat").getPath() returns c:\book\test.dat.
+getPath(): String
+getParent(): String
Returns the complete parent directory of the current directory or the file
represented by the File object. For example, new
File("c:\\book\\test.dat").getParent() returns c:\book.
+lastModified(): long
Returns the time that the file was last modified.
Returns the size of the file, or 0 if it does not exist or if it is a directory.
Returns the files under the directory for a directory File object.
+length(): long
+listFile(): File[]
+delete(): boolean
Deletes the file or directory represented by this File object.The method returns
true if the deletion succeeds.
+renameTo(dest: File): boolean
Renames the file or directory represented by this File object to the specified name
represented in dest. The method returns true if the operation succeeds.
+mkdir(): boolean
Creates a directory represented in this File object. Returns true if the the directory is
created successfully.
+mkdirs(): boolean
Same as mkdir() except that it creates directory along with its parent directories if
the parent directories do not exist.
FIGURE 12.6 The File class can be used to obtain file and directory properties, to delete and rename files and directories,
and to create directories.
Caution
\ in file names
The directory separator for Windows is a backslash (\). The backslash is a special character in Java and should be written as \\ in a string literal (see Table 4.5).
Note
Constructing a File instance does not create a file on the machine. You can create a
File instance for any file name regardless whether it exists or not. You can invoke the
exists() method on a File instance to check whether the file exists.
relative file name
Java directory separator (/)
Do not use absolute file names in your program. If you use a file name such as c:\\book\\
Welcome.java, it will work on Windows but not on other platforms. You should use a file
name relative to the current directory. For example, you may create a File object using new
File("Welcome.java") for the file Welcome.java in the current directory. You may create a File object using new File("image/us.gif") for the file us.gif under the image
directory in the current directory. The forward slash (/) is the Java directory separator, which
12.10 The File Class 475
is the same as on UNIX. The statement new File("image/us.gif") works on Windows,
UNIX, and any other platform.
Listing 12.12 demonstrates how to create a File object and use the methods in the File
class to obtain its properties. The program creates a File object for the file us.gif. This file is
stored under the image directory in the current directory.
LISTING 12.12 TestFileClass.java
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
public class TestFileClass {
public static void main(String[] args) {
java.io.File file = new java.io.File("image/us.gif");
System.out.println("Does it exist? " + file.exists());
System.out.println("The file has " + file.length() + " bytes");
System.out.println("Can it be read? " + file.canRead());
System.out.println("Can it be written? " + file.canWrite());
System.out.println("Is it a directory? " + file.isDirectory());
System.out.println("Is it a file? " + file.isFile());
System.out.println("Is it absolute? " + file.isAbsolute());
System.out.println("Is it hidden? " + file.isHidden());
System.out.println("Absolute path is " +
file.getAbsolutePath());
System.out.println("Last modified on " +
new java.util.Date(file.lastModified()));
}
}
create a File
exists()
length()
canRead()
canWrite()
isDirectory()
isFile()
isAbsolute()
isHidden()
getAbsolutePath()
lastModified()
The lastModified() method returns the date and time when the file was last modified,
measured in milliseconds since the beginning of UNIX time (00:00:00 GMT, January 1,
1970). The Date class is used to display it in a readable format in lines 14–15.
Figure 12.7a shows a sample run of the program on Windows, and Figure 12.7b, a sample
run on UNIX. As shown in the figures, the path-naming conventions on Windows are different from those on UNIX.
(a) On Windows
FIGURE 12.7
(b) On UNIX
The program creates a File object and displays file properties.
12.27 What is wrong about creating a File object using the following statement?
new File("c:\book\test.dat");
12.28 How do you check whether a file already exists? How do you delete a file? How do
you rename a file? Can you find the file size (the number of bytes) using the File
class? How do you create a directory?
12.29 Can you use the File class for I/O? Does creating a File object create a file on the disk?
✓
Check
Point
476 Chapter 12
Exception Handling and Text I/O
12.11 File Input and Output
Key
Point
VideoNote
Write and read data
Use the Scanner class for reading text data from a file and the PrintWriter class
for writing text data to a file.
A File object encapsulates the properties of a file or a path, but it does not contain the methods for creating a file or for writing/reading data to/from a file (referred to as data input and
output, or I/O for short). In order to perform I/O, you need to create objects using appropriate Java I/O classes. The objects contain the methods for reading/writing data from/to a file.
There are two types of files: text and binary. Text files are essentially characters on disk. This
section introduces how to read/write strings and numeric values from/to a text file using the
Scanner and PrintWriter classes. Binary files will be introduced in Chapter 17.
12.11.1
Writing Data Using PrintWriter
The java.io.PrintWriter class can be used to create a file and write data to a text file.
First, you have to create a PrintWriter object for a text file as follows:
PrintWriter output = new PrintWriter(filename);
Then, you can invoke the print, println, and printf methods on the PrintWriter object
to write data to a file. Figure 12.8 summarizes frequently used methods in PrintWriter.
java.io.PrintWriter
+PrintWriter(file: File)
+PrintWriter(filename: String)
+print(s: String): void
+print(c: char): void
+print(cArray: char[]): void
+print(i: int): void
+print(l: long): void
+print(f: float): void
+print(d: double): void
+print(b: boolean): void
Also contains the overloaded
println methods.
Creates a PrintWriter object for the specified file object.
Creates a PrintWriter object for the specified file-name string.
Writes a string to the file.
Writes a character to the file.
Writes an array of characters to the file.
Writes an int value to the file.
Writes a long value to the file.
Writes a float value to the file.
Writes a double value to the file.
Writes a boolean value to the file.
A println method acts like a print method; additionally, it
prints a line separator. The line-separator string is defined
by the system. It is \r\n on Windows and \n on Unix.
Also contains the overloaded
printf methods.
The printf method was introduced in §4.6, “Formatting
Console Output.”
FIGURE 12.8 The PrintWriter class contains the methods for writing data to a text file.
Listing 12.13 gives an example that creates an instance of PrintWriter and writes two
lines to the file scores.txt. Each line consists of a first name (a string), a middle-name initial
(a character), a last name (a string), and a score (an integer).
LISTING 12.13 WriteData.java
throws an exception
create File object
file exist?
1 public class WriteData {
2
public static void main(String[] args) throws IOException {
3
java.io.File file = new java.io.File("scores.txt");
4
if (file.exists()) {
5
System.out.println("File already exists");
6
System.exit(1);
7
}
8
12.11 File Input and Output 477
9
10
11
12
13
14
15
16
17
18
19
20
}
21 }
// Create a file
java.io.PrintWriter output = new java.io.PrintWriter(file);
// Write formatted output to the file
output.print("John T Smith ");
output.println(90);
output.print("Eric K Jones ");
output.println(85);
print data
John T Smith 90 scores.txt
Eric K Jones 85
// Close the file
output.close();
Lines 4–7 check whether the file scores.txt exists. If so, exit the program (line 6).
Invoking the constructor of PrintWriter will create a new file if the file does not exist. If the
file already exists, the current content in the file will be discarded without verifying with the user.
Invoking the constructor of PrintWriter may throw an I/O exception. Java forces you to write
the code to deal with this type of exception. For simplicity, we declare throws IOException in
the main method header (line 2).
You have used the System.out.print, System.out.println, and System.out.
printf methods to write text to the console. System.out is a standard Java object for the
console output. You can create PrintWriter objects for writing text to any file using print,
println, and printf (lines 13–16).
The close() method must be used to close the file (line 19). If this method is not invoked,
the data may not be saved properly in the file.
12.11.2
create PrintWriter
close file
create a file
throws IOException
print method
close file
Closing Resources Automatically Using try-with-resources
Programmers often forget to close the file. JDK 7 provides the followings new try-withresources syntax that automatically closes the files.
try (declare and create resources) {
Use the resource to process the file;
}
Using the try-with-resources syntax, we rewrite the code in Listing 12.13 in Listing 12.14.
LISTING 12.14 WriteDataWithAutoClose.java
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
public class WriteDataWithAutoClose {
public static void main(String[] args) throws Exception {
java.io.File file = new java.io.File("scores.txt");
if (file.exists()) {
System.out.println("File already exists");
System.exit(0);
}
try (
// Create a file
java.io.PrintWriter output = new java.io.PrintWriter(file);
) {
// Write formatted output to the file
output.print("John T Smith ");
output.println(90);
output.print("Eric K Jones ");
output.println(85);
}
}
}
declare/create resource
use the resouce
478 Chapter 12
Exception Handling and Text I/O
A resource is declared and created followed by the keyword try. Note that the resources are
enclosed in the parentheses (lines 9–12). The resources must be a subtype of AutoCloseable
such as a PrinterWriter that has the close() method. A resource must be declared and
created in the same statement and multiple resources can be declared and created inside the
parentheses. The statements in the block (lines 12–18) immediately following the resource
declaration use the resource. After the block is finished, the resource’s close() method
is automatically invoked to close the resource. Using try-with-resources can not only avoid
errors but also make the code simpler.
12.11.3
Reading Data Using Scanner
The java.util.Scanner class was used to read strings and primitive values from the console in Section 2.3, Reading Input from the Console. A Scanner breaks its input into tokens
delimited by whitespace characters. To read from the keyboard, you create a Scanner for
System.in, as follows:
Scanner input = new Scanner(System.in);
To read from a file, c